Other than ants, the most frequently occurring mob is the Rhino Beetle: there are five of them in the game. The least frequently occurring mob is the Stump Snail, re-spawning 96 hours (4 days) after you defeat it.
You can't fight the mobs yourself; only your bees can. One method for defeating mobs that works reliably for any mob which is defending a field (so everything other than the King Beetle, Tunnel Bear, Ants, the Cave Monsters, Stick Bug, and Rogue Vicious Bee) is to jump continuously. When you're in the air, the mobs want to retreat, but when you land, they want to attack, so they end up frozen with indecision. You do have to make sure your bees "know" to attack a mob before starting to jump - if the mob hasn't started attacking you, your bees might not recognize it as a threat.
Mobs have levels that are shown next to their label. If your bees are a lower level than the mob they're attacking, they can miss.