After a couple of games I've noticed that when chasing a fleeing hero, the Beast cannot successfully attack units that are retreating away. Here's how the attack cycle goes after clicking attack.
1) Beast moves up to the target
2) Attack animation begins and Beast stops moving
3) Enemy exits attack range and attack animation is canceled
4) Beast starts chasing after a brief pause
5) ???
6) Repeat 1-5 until Beast dies. No PROFIT.