You can pick really interesting cohorts for your character.
My 15th level Wizard (NG) has this cohort:
Strix the Giant Owl
Gargantuan Magical Beast
Hit Dice: 9d10+45 (94 hp)
Initiative: +2 (Dex)
Speed: 10 ft., fly 70 ft. (average)
AC: 18 (-4 size, +3 Dex, +10 natural)
Attacks: 2 claws +12 melee, bite +7 melee
Damage: Claw 2d6+12, bite 2d8+6
Face/Reach: 20 ft. by 40 ft./10 ft.
Special Qualities: Superior low-light vision
Saves: Fort +11, Ref +8, Will +5
Abilities: Str 34, Dex 15, Con 20, Int 10, Wis 14, Cha 10
Skills: Knowledge (nature) +7, Listen +17, Move Silently +10*,
Spot +12*
Feats: Alertness, Flyby Attack
Climate/Terrain: Any forest, hill, mountains and plains
Organization: Solitary, pair, or company (2-5)
Challenge Rating: 3 (225% XP-award)
Treasure: None
Alignment: Neutral good
Advancement: 5-8 HD (Huge); 9-12 HD (Gargantuan)
Strix speak Common and Sylvan.
Combat
Strix attacks by gliding silently just a few feet above his prey and plunging to strike when directly overhead.
Superior Low-Light Vision (Ex): Strix can see five times as far as a human can in dim light.
Skills: Strix receive a +8 racial bonus to Listen checks. *He also receive a +4 racial bonus to Spot checks in dusk and darkness; when in flight, he gain a +8 bonus to Move Silently checks.
Essentially Strix is a giant owl with 5 extra HD. This makes him a 9HD creature. When we add 3 (arbitrary number from the DMG on account of Strix being a magical beast) to this, Strix becomes an 11th level cohort. Which, by the way, is exactly what my wizard's leadership score can manage.