There are many monsters in minecraft.

1) Zombie-- usually regarded as the easiest monster to kill. Burns in daylight and drops rotten flesh. Yum! Spawns in darkness

2) Spider-- Basically a giant spider. Known for its abilities to climb walls and attack aggressively.

3) Creeper-- The most well known monster in Minecraft. Explodes when approached and is the easiest way to get gunpowder.

4) Enderman-- Based off of "Slenderman." Agressive when stared at, and can teleport. It recieves damage when touched by water or rain. Regarded as the deadliest OverWorld monster.

I know that there are many, many more misters out there. These are the most common. Please leave suggestions in the comments below.