I'm partial to wolves. They have 4 moves, so they can tag along with cavalry, and they have the ability I call 'Chomp!'. Once the hero engages a tough monster, I wait for the bite to recharge, and for the wolf to get two turns to the enemy's one. Then the wolf dashes in, bites the monster and dashes out before the monster can react. EXACTLY as wolves do in real life.
As the wolf grows, it becomes less fragile, but unfortunately its bite grows no more powerful. Ah well.