Monster Rancher was a weird one. The whole idea of monsters hidden in our CDs was a cool gimmick. And I love their monster designs, even if it was reshading the same monsters.
The gameplay was hit or miss, I guess. Pokémon plays out like a typical turn-based RPG; Monster Rancher was management + action based. For me the fun part was fighting against time to get a perfect monster, it was surprisingly hard to do so (limited lifespan, the need to train good coaches beforehand, etc.)
A few games actually had different models for the sub-breeds, besides rehashing the old mons. Like MR4: