Lone wolf, 8r, Common
Beast
Attack: 10n
N is #rares in opponent's hand
As it stands now, this card could deal 80 damage (without Beast keyword effect). That's highly unlikely, of course, but an opponent having as many as 5 on hand isn't unheard of with careful keyword usage.
I don't feel that a Common card should be capable of so much. Currently, the most a Common can do is 16 in the most favorable situation, iirc (Goblin Saboteur). Most Commons with variable damage peak at 12.
My suggestion would be to give it a base attack value and add a multiple of N where N is the number of Rares in the opponent's hand over 2 with a cap of 3. (That is, the opponent needs to have 3, 4, or 5+ Rares before N adds damage.) Perhaps Attack: 4 + 5N or 6 + 4N. Having a damage cap is important if this is going to remain Common.