Thanks King Serperior! SlowMo isn't actually too bad. Next chapter I teach him a move that boosts his usefulness by a lot. I tried teaching him Surf, but I have to wait until he is a Rhydon.
I got pretty lucky when trying to find a Rhyhorn to fit the requirements though. A Speed reducing nature+Rock Head should have taken a while to find. But I found a suitable Rhyhorn on only the fifth try.