Like others have said, the modding isn't affecting Mojang's ability (or the legality of) to add new mobs to the game, even if those mobs have already been added the Mo' Creatures or another mod. The only reason the creator of Mo' Creatures is listed in the credits is that Mojang voluntarily got help from him to work on the horse mob.
Additionally, Jeb's statement about the turtle only meant that he wanted to add his own original spin on it
I believe the shulker was that original spin on it, because it resembles the turtle in an abstract way. But I could of course be wrong on that.