the spell works only on humanoid creatures
many people say that they can cast it upon their eidolon
in which case is an eidolon humanoid?
an eidolon is an outsider. can outsider be humanoid?
i doubt this is in quadruple form but if the eidolon looks like a centaur (adding some arms) ... centaurs count as monstrous humanoid. but without arms it is e.g. a dog. does a dog-eidolon count as an animal?