Alright, I love this game but the monkeys and Leech Monsters are garbage!

[BloodStarvedHoonter]

This game is good but, now its getting to the point where I am beyond frustrated because of the monkeys and leech monsters, both spam you like crazy, especially the monkeys! The leech monsters are annoying as ♥♥♥♥! Then not to mention if you kill the damn things with acid rounds, they explode and bring you down to danger, and because the health items are so damn rare even in normal, have fun trying to get Rebecca or Billy back to Fine. ♥♥♥♥ these enemies, Capcom, they are not tense, they are not fun to fight, they are ♥♥♥♥♥♥♥t! RE1 Remake, did everything right, I can pass zombies, save health and ammo. But here, nah, the monkeys spam attack the crap out of you and there is no way to get out, come back through the same door and they are right there waiting at that door. ♥♥♥♥♥♥♥, these enemies are at the point where I am considering to just not play the game anymore and just go back to RE1 Remake, the much MUCH better game overall. UGH!

[James.Havoc]

Well, all that might be the reason, why those enemies (unlike Licker&Hunter) never made it into another game from the franchise. (Except Umbrella Chronicles).

On the other hand you might try to fight the leech Man with fire instead of acid. That way he wont explode. Fire grenades do the job very well, but I would suggest using the molotovs. (Yes, I know, they are very awkward to throw). There are plenty of them to make and they kill the Leech usually in 2-3 hits.
As for the monkeys...well, you only get to see them in the training facility. After that they never appear again.
Also if you do it right, you only have to fight the three monkeys in the area with animal statues. And there you can just wait at the start an kill them off one by one with the shotgun.

[Lanzagranadas]

Instead of wasting health and resources fighting Leech Men, you could learn their patterns and start avoiding them. They show up just in specific parts of the game and can be avoided, save often and plan your route so you don't have to go where they are multiple times. In many cases, you can simply run past them and leave the room before they actually mutate into their attacking form, so the first time is basically a free dodge. If you don't have anything decent to fight them (basically: Incendiary rounds), then it's better to not fight them at all. Molotovs work but are risky, and if you don't get the right timing you're going to get hit anyway.

A bunch of annoying enemies in a RE game isn't anything new or exclusive to this game, but none of them are impossible to deal with.

Originally posted by James.Havoc:

Well, all that might be the reason, why those enemies (unlike Licker&Hunter) never made it into another game from the franchise. (Except Umbrella Chronicles).

They couldn't have many chances of showing up in other games because RE0 was the fifth (and last) released title in the main series, and after that we got RE4 which completely rebooted the enemy roster.

Besides, many of the creatures introduced in RE0 are essentially leftovers of the primitive T-Virus experiments to test the virus effects on different animals; lore-wise, they (and Mimicry Marcus of course) wouldn't have made sense outside RE0 anyway (though we do have a Leech Man in RE Outbreak). Creatures like Hunters are manufactured and mass-produced BOWs so it's reasonable that they would have multiple appearances throughout the franchise.

[James.Havoc]

Yet we got Lickers back in RE5. If it was for bringing back old creatures, Capcom always found ways to do that. And it never mattered, if it would make sense or not. And no, some poorly written document inside the game is no excuse.

[Lanzagranadas]

Because there happens to be an old Umbrella Facility in RE5 where experiments with Progenitor and T-Virus were conducted and so researchers happened to produce Lickers there. You don't just randomly see T-based creatures in games that have plot-wise nothing to do with that virus appearing out of nowhere, unless the lore can explain why they're there.

Eliminators were never mass-produced. They were early T-Virus experiments on some monkeys before Marcus decided to start using humans as test subjects, which allowed Umbrella to produce much more advanced BOW species, rendering Eliminators and other experiments outdated.

All RE games have some enemy that never returned anyway (outside spin-offs and retellings of said games they belong to):
RE0: Eliminator, Plague Crawler, Lurker
RE1: Neptune, Chimera, Wasp, Adder
RE2: Ivy
RE3: Brain Sucker, Drain Deimos
RECV: Bandersnatch, Albinoid

It has nothing to do with enemies being good or bad. Sometimes there's just no reason to bring an enemy back because there's probably a new enemy already filling a similar role, so they would serve no purpose. Especially from RE4 onwards since RE games are no longer centered around the T-Virus and its variants so they have to bring new enemies.