The rules say an Eidolon appearance is up to the summoner but always looks like a fantastic creature.
If I am on an urban adventure and I summon my Eidolon and try to get it to appear Human (or as a half elf maid using the Fey template in my case ) what would be the chances of somebody noticing something is wrong? And could you use the disguise skill to make it blend in. granted certain evolution cannot be picked(Wings, tentacles,etc) cannot be used if you are trying to get your eidolon to blend in.
|Tiny Coffee Golem|
I'd say that any "fantastic creature" that can easily be disguised as a mundane humanoid, isn't fantastic enough. It should be possible, but the DC needs to be pretty high.
That's my opinion anyway. The Disguise skill description mentions no cases of disguising oneself as a completely different type of creature, and only has the default -2 penalty for disguising as a different race.
The 'always looks like a fantastic creature' is fluff.
If you're not choosing wings, or claws, or bite etc, and instead wanted to summon an Eidolon with all skill boost, aka, the Librarian, it shouldn't be hard to disguise the one feature that would 'possibly' give her away. (the glowing rune.)
Regardless there are no rules I can think of saying that when using the disguise skill Eidolons receive a penalty to the skill.
p.s. buy the +8 skill bonus with an evolution point too, to really boost that skill.