For those that don't know or remember, Chaos is the blobby water monster from Sonic Adventure, mainly known today for allowing artists to use an existing character to create tentacle porn. There are quite a few images featuring Chaos on this site, and few of them seem to be consistent: monster, chao, humanoid, chao + humanoid, chao + humanoid + goo, etc. The various Sonic wikis are a little more consistent, from a mutant Chao to a mutant Chao/water beast to an ascended Chao. Chao probably belongs somewhere in there, but would humanoid or goo be just as appropriate?

Depends entirely on how they're drawn.
Humanoid and goo_creature are applicable in most cases. Maybe monster too, but that's debatable; the monster tag is kind of subjective.

I can see some posts where tentacle_monster definitely applies, though.

For example:
post #34345 - goo_creature, tentacle_monster
post #806273 - goo_creature, humanoid

I'm pretty sure Chaos is some kind of water elemental, but I don't know if e621 has that as a tag.

Thirtyeight said:
I'm pretty sure Chaos is some kind of water elemental, but I don't know if e621 has that as a tag.

It exists, but hasn't been used much. Such creatures tend to get tagged as goo, on the basis that they look solid enough to be closer to that than water. With a few exceptions, such as post #495408.

Characters aren't going to get implicated to species, because they can be portrayed as other species. You just gotta tag it as it comes up per image.

...Also, those "Chao" ones don't look at all like chaos.

He's technically based off of an aboeba, according to my brother who is a huge sonic the hedgehog fan boy.

Technically a mutated Chao, but that's more a canon argument than a tagging one.