"There's a word for someone who brings too much gear to battle: 'survivor.'"
—Zada of Slab Haven

Zada's Commando

First strike
Cohort — {T}, Tap an untapped Ally you control: This creature deals 1 damage to target opponent or planeswalker.

The List

Oath of the Gatewatch

First strike
Cohort — {T}, Tap an untapped Ally you control: Zada's Commando deals 1 damage to target opponent.

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].)