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?

Also, please note that monstrous humanoids and humanoids are different. You can't use charm person, enlarge person, hold person, etc. on a monstrous humanoid, despite the presence of humanoid in their type name.