It seems like there is some inconsistency with how alternate_species are handled on the site.
According to the wiki alternative_species are meant for when a character is drawn as a different species, yet we have pages after pages of pokemon drawn as a humanoid using the tag humanoidized, which implicates alternate_species.
It doesn't help with the matter that one of the thumbnails for humanoidized is a generic flareon drawn as a humanoid. post #247865
The same inconsistency can be found in the humanized wiki page with a generic chespin drawn as a human. post #385656
To my knowledge species aren't characters, so a species drawn as another species should not have these tags at all.