Small step, useful reward - Boss fights in Pre-Hardmode

A guide to help you with the pre-hardmode bosses, what summons them and what their loot is useful for.

The everwatching one - Eye of Cthulhu

The Eye of Cthulhu may be the first boss you'll encounter. It has 2800 HP and deals a good amount of damage, around 15 per hit. At the amount of 200 HP a player needs to summon it automatically it is not that harmless.
It has 3 requirements to be summoned:
-200 HP or more
-10 defense or more
-3 NPCs or more inhabit your town
From time to time while fighting, it will spawn a few enemies in, Servants of Cthulhu. Those are little Demon Eyes that could be easily killed due to low HP.
Also it will attack you from time to time by charging at you with a horrifying scream. Can easily be dodged if you jump at the right time.
Here's the critical part: At >1400 HP (>50%) it will change it's form and grow beast-like teeth. At this status it's damage will raise to 23, it will ONLY attack by charging from this moment on, but it will be easier to kill due to it's defense dropping from 12 to 0.
To survive the fight, you should at least own Silver armor (Gold would be best at this progress) and also a good broadsword. A Silver or Gold Broadsword should be enough. A nice ranged weapon like a Silver Bow can also help dealing with the 2nd form. Best use Flaming Arrows due to it not being immune to the On Fire! debuff.
On defeat it will drop 3 Gold Coins, 30-87 Demonite Ore, 20-49 Unholy Arrows, 1-3 Corrupt Seeds and 5-15 Lesser Healing Potions. Also it will have a 10% chance of dropping the Binoculars, allowing you to increase your view range upon use.
The Demonite can be used to craft the War Axe of the Night or Light's Bane at that moment due to the requirement of Shadow Scales for every other part of Demonite equipment.
Also it can be spawned manually using Suspicious Looking Eye, which can be crafted at a Demon Altar using 6 Lenses.

The splitting one - Eater of Worlds

The Eater of Worlds is probably the easiest boss to defeat. It has the AI of a worm and is therefore easy to notice when attacking. It is built of 100 seperate segments with 150 HP each, bringing it's total to 15000 HP, the highest HP a pre-hardmode enemy can own. As soon as one segment is destroyed, it will split up at that destroyed segment and become two enemies of different size. Each of the 100 segments needs to be destroyed until the Eater of Worlds is defeated.
Like any worm, it is weak against a weapon that can hit multiple enemies with one use. A Ball O' Hurt would be appreciated.
Also a good armor is required. Silver or Gold should be enough for the fight.
It can be summoned by destroying 3 Shadow Orbs in the Chasms of the Corruption. Also it can be manually summoned by using Worm Food made of 30 Vile Powder and 15 Rotten Chunks at a Demon Altar. But it will only spawn within Corruption, nowhere else.
It's damage depends on the segment you're hit with. The head will do 22 damage, but it has got only 2 defense. The middle segments do 13 damage, but have 2 more defense than the head and the tail deals only 11 damage, but is the hardest part to defeat with 8 defense. It's drops depend on the segments. Every segment will drop 3 Silver Coins, and at 50% chance 2-5 Demonite Ore or 1-2 Shadow Scales. Also, defeating it completely will make it drop 20-59 Demonite Ore and 5-15 Lesser Healing Potions.
This loot can prove useful due to it dropping Shadow Scales which can be used to build Shadow Armor. But also you can create a Nightmare Pickaxe with it to mine Hellstone in the Underworld.

The thinking one - Brain of Cthulhu

If your World has generated with a Crimson instead of a Corruption, the Eater of Worlds won't be able to spawn. Instead, the Brain of Cthulhu takes it's place.
It might be counted as a "weak" boss due to it's just 1000 HP, but it's pretty tricky to defeat.
It will spawn a lot of little enemies around it, attacking the player. Before these small enemies are defeated, the Brain will stay invulnerable! No matter whether or not the small enemies are defeated, do not touch the Brain that's teleporting around the room! Touching it will cause a lot of damage to the player.
It can be spawned by going down into the Crimson and smashing 3 Crimson Hearts with a hammer. Also it can be spawned using the Bloody Spine in the Crimson, which can be crafted using 15 Vertebrae at a Crimson Altar.
It will drop 30-80 Crimtane Ore, 30-50 Tissue Samples which can be used for crafting the Deathbringer Pickaxe, the Meatball (Flail), as well as the whole set of Crimson Armor.

The guarding one - Skeletron

Skeletron is probably the hardest pre-hardmode boss to defeat. It is built of 3 parts: His head and his 2 hands. His hands each have 600 HP, 14 defense and deal 20 damage on contact. His head owns 4400 HP and 10 defense while dealing massive 32 damage on contact.To defeat this boss, you need a good armor like Shadow Armor, but also a good equipment to be able to evade him. Rocket Boots and a Grappling Hook (Spectre Boots and Ivy Whip should be even better) should be a minimum of equipment to fight him.
Also a mighty blade might be good to defeat him. Either use Grass Blade due to it's high range or if already entered Underworld, you can use a Fiery Greatsword due to it's high damage.
Skeletron can be spawned by talking to the Old Man at the Dungeon at Night. You need to kill Skeletron to enter the Dungeon's chambers. Also, after Skeletron is defeated, the Dungeon Enemies will sometimes drop a Clothier Voodoo Doll. Killing the Clothier when equipped with it will spawn Skeletron again.
When entering the lower parts of the Dungeon before Skeletron is defeated, Dungeon Guardians will spawn and instantly kill you due to 9000 damage dealt.
He only has 2 attacks: He uses his hands to attack you or he drops to the floor, spinning his head wildly. His attack is his weakness: When spinning, his head's defense drops to 0.
But this boss is different: To defeat him, you don't have to defeat each segment seperately, you only have to destroy his head! Destroying his head will cause an instant death of the hands.
But the tricky about this boss: He has to be defeated within one single night. When still alive at sunrise, he will only take 1 damage and instantly kill any touched player. Waiting 'til next night won't help.
He might drop the Skeletron Hand, a Grappling Hook that can be thrown twice at the same time, but can only latch onto one surface. Also he might drop the Book of Skulls, a useful magic weapon. Defeating him makes the Dungeon accessible. The items found in the Dungeon will be worth something.
Also defeating him unlocks the Clothier in your town.

Addition: The slimy one - King Slime

The King Slime deals most damage of all of the pre-hardmode bosses. It might only have 2000 HP, but it deals unbelievable 40 damage on contact. It will move at regular Slime speed and it can spawn ANYTIME while daytime on grass blocks on the surface and outside of the inner third of the map. King Slime is the only boss enemy that can't travel through blocks, which makes trapping him easy.
You should have a good ranged weapon like Water Bolt, Magic Missile, Musket or Gold Bow. It is required to stay out of it's range to defeat it easily.
It won't drop anything useful, but it will drop one vanity item, a part of the Ninja Outfit.
Also it is probably the most expensive to summon manually due to it needing 99 Gel and a Crown crafted from a Ruby and 30 Gold Bars. So better hope to encounter it randomly then summoning it manually to keep your Gold.

[geeked up playing xbox all day]

This had no use to me, but keep on writing guides, this one is amazing ^_^

[2Chaac]

thanks nick, that really helped =D

[Nick EX]

@Rage Against The Nix: Sry, forgot to update. Thanks for the hint.
@Tyguy60: The WoF is considered as Hardmode for me due to the fact defeating it activates Hardmode. Look at my Hardmode guide for that.

[Nix Kyrie (Ky)]

Updates to bosses post 1.2: Eye - Rare chance to drop the binoculars, which can allow you to increase your view range by right clicking, 10% chance to drop trophy.
EoW - Rare chance to drop Eater's Bone, which summons a small eater pet. Currently bug and drops Blend-O-Matic instead, despite the hotfix meant to patch that. 10% to drop trophy.
Skeletron - Chance to drop Skeletron's Hands, pre-hard mode version of the Dual Hook. Chance to drop Skeletron Mask, a vanity helmet resembling Skeletron's head. Chance to drop a spell book (I've forgotton the name, something along the lines of 'Flaming Skulls' or 'Book of Skulls') which casts small flaming skulls. 10% chance to drop Skeletron trophy. Dungeon mobs have a small chance of dropping the Clothier's Voodoo Doll. When in your inventory, your attacks deal damage to the Clothier, and once you kill him Skeletron will be summoned. Best used at night to prevent Skeletron's Gaurdian status from activating.

[JoeyFreakyBob]

I hate where in the new update, he just spawns automatically when im not ready yet.

[2Chaac]

Where is the Wall of Flesh???

[Mjvucci]

@NickEx:
You are right that is is not that useful for the EoW. but I meant that it is useful against any mob like zombies or any everyday mob. Sorry about the confusion.

[Nick EX]

@mjvucci:
thx for the hint to the mistake I made with my guide. But using a Bow against EoW is not a good idea yk. For Skeletron it can prove useful, but for EoW I recommend Ball O' Hurt, Light's Bane or any Spear or Trident due to the high chance of hitting multiple segments.

[Mjvucci]

NickEX you can also craft the demon bow with out getting shadow scales. the bow is useful against the EoW and Skeletron too

[Nick EX]

The most considered easiest boss is Eater of Worlds. It's seperate segments do not have much HP, but when a segment is destroyed, it splits up at that point, so better be careful.