As far as I can tell, there's no way of assuming the shape of a monstrous humanoid with the Pathfinder rules. Polymorph covers animal, humanoid, or elemental. Greater Polymorph adds dragon and plant. Beast Shape gets animal and magical beast forms. Alter Self just gets humanoid.
I also find it weird that druids don't get Beast Shape. I was thinking that Monstrous Humanoid would make a decent Wild Shape ability, but there's no possibility of that anywhere in the roles that I can see. (Why
restrict druids from becoming centaurs, for example?) While druids do get, essentially, Beast Shape in their wild shape ability, I don't see any particular reason to restrict the spell from them--or even from rangers.
What do folks here think?