Monster Rancher doesn't have RPG elements. It's purely monster raising sim game. There's no over-world to explore and very little plot. You stay on a farm raising your monster, feeding and training it and entering it in tournaments to earn money and win prizes that unlock better monsters. The monsters have a limited lifespan so you have to breed them through multiple generations.
SMT is closer to Pokemon, it's an RPG with a story. Unlike Pokemon you have a player character that fights and levels up along side the monsters you recruit. The early SMT games (1 and 2) have very limited monster customization, the monsters don't grow, so you have to fuse/discard them in favor of stronger late game creatures. Later SMT (3, 4, so on) do let monsters level up and grow, so it's possible to make an early game wimp into an end game beast with enough grinding and min/max autism.
I haven't played any Digimon games, but from what I understand they're also RPGs in the same vein as SMT and Pokemon. Also for more Pokemon-like games look into the Dragon Quest Monsters series.