2 eyes
Because most creatures on e621 have exactly 2 eyes, this tag should probably only be used when it's in conjunction with some other number of eyes (1_eye or some number of multi_eyes from 3_eyes upward) — for example, a 2-eyed character undergoing some transformation and having their eyes fuse into 1 eye or split into several eyes.
This tag has been aliased to eyes (learn more).