Any thrall can apply the bleed debuff if you equip them with a weapon capable of it. Because stats don't contribute to bleed damage, just pick the thrall you like the most all around. The best animal follower that applies bleed is the sabretooth tiger. While the crocodile applies the bleed debuff the most often (with every attack), the sabretooth deals out much more damage. Bleed's number one benefit outside of damage is pausing heal, so pick the follower that can deal the most damage even if they apply the bleed debuff less regularly.