Shadowfax, Lord of Horses

{3}: Exile another target creature you control, then return it to the battlefield
under its owner's control.
Whenever another creature enters the battlefield under your control, you may pay {g/w}. If you do, put a +1/+1 counter on
it. If it's a Unicorn, put two +1/+1 counters
on it instead. ({g/w} can be paid with either {g} or {w}.)