Cohort

Cohort is an ability word introduced in Oath of the Gatewatch and associated with allies. The activated ability requires tapping the ally creature and another ally.

Description
The cost of each cohort ability (tapping, and tapping another ally) is always the same but the effects differ wildly. Because the cost includes undefined, the Ally that has the cohort ability has to have been under your control since your most recent turn began. It can't have "summoning sickness." But the other Ally, the "an untapped Ally you control," could be one you just cast.

Rulings

 * 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 undefined [the tap symbol] .)