Munda's Vanguard — ruling 1

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].)

Munda's Vanguard : règles, fonctionnement et interactions (MTG)

Munda's Vanguard is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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

How it works : Munda's Vanguard

Original English text — Wizards of the Coast / Scryfall

#1January 22, 2016· Wizards of the Coast
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 [the tap symbol].)
No other card is named in this ruling.