Symbiotic Evolution

Whenever a Beast you control attacks and is blocked, you can tap another creature you control with a less converted mana cost than the blocked creature. If you do, untap the blocked creature and remove it from combat, then put a +1/+1 counter on it.
The Lummox are a race of herbivores who are unable to see for themselves, so they regularly enlist the aid of smaller and faster species in order to know if the area ahead of them is safe or not.