"Do not let yourselves fall now. Zendikar is depending on us-on you!"

Munda's Vanguard

Cohort — {T}, Tap an untapped Ally you control: Put a +1/+1 counter on each creature you control.

Magic Online Promos

"Do not let yourselves fall now. Zendikar is depending on us—on you!"

Oath of the Gatewatch

Oath of the Gatewatch Promos

To activate a cohort ability, the Ally with that ability must have been under your control continuously since the beginning of your most recent turn. Informally, it can’t have “summoning sickness.” However, the other Ally you tap can be one that just came under your control. (Note that tapping the second Ally doesn’t use {T} [the tap symbol].)