Also things that go bump against you, and want to take away your precious hearts.
There are 3 categories of monsters -- regular ones, minibosses, and bosses.
The minibosses are scattered throughout the levels and are just harder than normal monsters -- they have more hit points and have special abilities like casting magic or teleporting.
The bosses are located at the end of a stage (not all stages have a boss), and are much harder to kill. Their special abilities are not affected by the Silence spell.