I think there should be more class exclusive companions, I was thinking 1 for level 25 where you will be required to defeat a certain boss in an instance and 1 for level 50 where you will be required to complete a whole dungeon.

Level 25: (Promotions: LV39, LV61)
Madame Bonita(Skeletal Enchantress) (Skeletal Evoker) (Skeletal Necromancer)
Sir Lance(Dog Fencer) (Dog Fencing Artisan) (Dog Fencing Master)
Bearonica(Polar Bear Warrior) (Polar Bear Guard) (Polar Bear Battle-Master)
Librarian Leo(Unicorn Librarian) (Unicorn Cleric) (Unicorn Negotiator)
Sherriff Fangs(Rattlesnake Shootist) (Rattlesnake Sharpshooter) (Rattlesnake Shooting-Star)

Level 50: (Promotions: LV64)
Dezrael(Accursed Spirit) (Forsaken Spirit)
Katarina(Capoeira Cat) (Combatant Cat)
Beelzah(Krokotopian Spearwoman) (Krokotopian Commander)
Joanna of Cluck(Dove Warrior) (Dove Commander)
Naughty Johnny(Rat Prankster) (Rat Slinger)