Irrelevant. To quote the Bestiary itself (where the Monster feats appear):
Note the phrase after "although." If you qualify for the feat, then you can take it- nothing says you have to be a "monster" to do so (and even if it did, guess what: most of the races have listings in the Bestiaries and therefore count as "monsters" in the literal sense).
Improved Natural Attack lists the following Prerequisites: Natural weapon, base attack bonus +4.
Nothing on there about race or creature type. Therefore, if you have one or more natural weapons and a BAB of +4 or higher, you can take the feat. The only kind you can't take it for is an Unarmed Strike, and that's because the feat text itself says so.