Druids have a wolf as an animal companion. This wolf receives all buffs that the druid gets, but not debuffs. Also, it has better AI than a normal wolf, respawns 5 minutes after dying, and cannot despawn. As its master gains levels, the wolf gains powers. Note that HP and damage buffs replace the previous ones, but other things do not.
3rd-5th level: 30 HP, +1 damage, +2 blocks per second movement
6th-8th level: 40 HP, +2 damage, immunity to animal controlling
9th-11th level: 50 HP, +4 damage, even better AI
12th-14th level: 60 HP, +6 damage
15th-17th level: 80 HP, +8 damage, +4 total blocks per second movement
18th-20th level: 100 HP, +10 damage