What exactly makes an outer god?

[Thomas Devlin]

The outer gods seem to come in all shapes and sizes. The Formless mother seems to be some kind of omni present extra dimensional being, you commune with her by piercing her body and making her bleed, which is literally done by stabbing at the air from any location, The Greater Will sent the Elden Beast to the lands between in the form of a meteor, so we don't know what it is, but it is probably physically located somewhere in outer space, and The Fell God lives inside of the fire giants, and it's implied that you actually kill it when you kill the last fire giant, so the actual definition of an outer god is pretty fast and loose, it can pretty much be anything.
Here's where things get tricky, to my knowledge the only outer gods that are explicitly stated in game to be outer gods are the unnamed outer god of the death birds, and The Formless Mother, seeing as we don't have any concrete definition on what an outer god is that makes the speculation even more speculative, since we don't even know for certain that half of the outer gods are even outer gods at all.
It's debated if The Great Serpent is an outer god or not, it just kind of seems like a really powerful snake, however The Rot God was literally in the lands between and was physically defeated and sealed away by the blind swordsman. We never see it in game despite traveling to the location it was sealed away in, and the scorpion stinger description implies it was crafted from The Rot God's tail, so there's evidence that it might have even been killed, so when you look at the evidence The Rot God really doesn't seem like an outer god at all, despite generally being classified as one, and if it is, then The Great Serpent could easily be classified as an outer god too, despite not really seeming like one

[Lichtgestalt]

What about Malenia, Goddess of Rot? Was the Rot God inside of Malenia all that time but she kept it from breaking free until we hit her enough?