Centaur Warleader

Whenever Centaur Warleader enters the
battlefield or attacks, raise a red and green
Centaur Army. (To raise, put a +1/+1 counter on
an Army you control. If you don't control one, first
create a 0/0 creature token of the specified color and
type.)
As long as you are attacking with two or more Centaurs and/or Armies, attacking Centaurs and Armies have flanking.