okay how about,
Empty the cages-
The beast master removes cages from cities, adding strength(promotions?) to himself for each animal freed.
wolf, +1 str or enables sprint w/+35% vs meelee
bear, +2 strength
elephant, +2 strength, +35% vs mounted
panther, +1 strength, +35% vs archers, or stealth in jungles
lion, +1 strength, perm courage and loyalty effect
gorrilla +1 strength, +25% city attack
spider +2 poison, enemy units become entangled
(units attacking the beastmaster can't withdraw, if it withdraws from another unit that unit can't move)