Going way out on some limbs here. A primate/primitive ungulate don't need to produce a hybrid or fertile offspring since obviously aliens mixed chimpanzee and

Going way out on some limbs here. A primate/primitive ungulate don't need to produce a hybrid or fertile offspring since obviously aliens mixed chimpanzee and pig DNA to create us, a race of slaves to mine gold for them. They also removed our ability to synthesize vitamin C in the process as a method of control.