Somewhat related threads:
topic #26568 (Implication on animal genital tags, tag defs)
topic #16000 (Tag defs)
topic #6381 (Tag defs)
topic #1570 (Implication multi/hemi)

Basically, don't we normally use the singular form of a word by default, and plural only in cases of ambiguity? Granted, 99.9% of the time, if you see half of a pair, you see the other for these tags. I guess it makes sense to use the plural form but not sure which is actually official tag. Shouldn't the different spellings be aliased like door/doors, canines/canine, and so on?

Typically, we also use the plural version for anatomical features that normally come in pairs. In an inversion of the normal practice, if we see one eye, we can pretty much safely infer there's a second eye. Sure, there are cases where there's 1_eye, but that's hardly different from other tags describing deviations from the default number, such as 3_eyes.