Jellys move slowly and can only hurt you if you touch them. They're easy to kill.
Blue Noctos fly around slowly and occasionally stop for a rest. They're hard to hit because they fly on angles. You will see the Pink Noctos further on in the game. They fly faster and take more than one hit to kill, unlike the Blue ones.
The Rattus runs around in short bursts, then spins in one spot and runs again. They take more than one hit to kill, and they often drop hearts.
Yellow Loopers stay in one spot until you walk past their sight, then they charge at you. The Green Loopers will slither around the room until it sees you head-on, and then charge. Black Loopers slither around the room and if you cross their path from any direction they'll charge. All Loopers take more than one hit to kill them.