Firesower Griffin

Flying
Steed (Whenever this creature attacks, once per turn, you may tap an untapped Knight creature you control. If you do, this creature gains all keyword abilities of the tapped creature and gets +X/+Y until end of turn, where X is the tapped creature's power and Y is its toughness.)
Whenever Firesower Griffin becomes a steed, it deals 1 damage to each opponent.