Actually, they are the ones we regard as normal. Ghost/sim, genie/sim, IF/sim and vampire/sim. That may sound a little elementary, but I asked because I was re-writing the rules to an old challenge and one of the couple has to be a sim. With the limitations of the challenge, I wanted to know with which creatures that sim could reproduce without resorting to cheating or fancy footwork. I did discover a legitimate way to have a mummy/sim hybrid, by impregnating the female sim and then having her sleep in the requisite sarcophagus, but I decided that was too complex for this challenge. It also appears that both mummies and simbots can have families on story progression, but of course that doesn't help in this challenge where the active family is the one trying to bring in the creature's genes.
As for the creature/creature hybrids, I'm sure any of those four could reproduce with any of the remaining three. It's too bad they can't all reproduce normally. That was a fun part of Sims 2.