Minecraft Mobs

Utility
Utility Mobs are Mobs that are created by players and will attack other mobs. Utility mobs usually attack Hostile mobs, but can sometimes also attack Neutral Mobs, Passive Mobs, or a player if provoked. A notable exception is the Wither, which attacks all mobs except for undead mobs.

Passive
Passive Mobs are Mobs that are harmless to players and some will flee if attacked.

Neutral
Neutral Mobs are mobs that will attack a player only when they are provoked by a player. However, they can sometimes attack other mobs as well.

Hostile
Hostile Mobs are mobs that will attack a player when he/she gets within a certain range of them. Most hostile mobs will attack when a player gets within 16 blocks of them without any obstructions, except for spiders, which could see through obstructions before P.C. Version 1.8, and ghasts, which can detect a player from 100 blocks away.

Boss
Boss Mobs are special types of Hostile Mobs that have more health than most hostile mobs, have a larger detection range, and have a Health Bar and name displayed at the top of any nearby player's screen if a player gets within range of them.

Unused
Unused Mobs are Mobs that exist in the Minecraft source code, but cannot be spawned in default survival without the use of commands, but the Zombie Horse has its own Spawn Egg.

Trivia
Some mobs can ride on others, such as a spider jockey, which can rarely spawn. You can spawn jokey that does not relate to zombie or spider by using commands. /summon phantom ~ ~ ~ {Passengers:[{id:drowned,HandItems:[{id:trident,Count:1b}]}]}