Ornery Tumblewagg (171) [Outlaws of Thunder Junction]

At the beginning of combat on your turn, put a +1/+1 counter on target creature. Whenever this creature attacks while saddled, double the number of +1/+1 counters on target creature. Saddle 2 (Tap any number of other creatures you control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)