Aven Eternal

If you don't control an Army, the Zombie Army token you create enters the battlefield as a 0/0 creature before receiving counters. Any abilities that trigger when a creature with a certain power enters the battlefield, such as that of Mentor of the Meek, will see the token enter as a 0/0 creature before it gets +1/+1 counters.

Amass Orcs works the same way, except you create a 0/0 black Orc Army creature token if you don't control an Army. If the Army creature you chose isn't already an Orc, it becomes an Orc in addition to its other types. By combining cards with amass Orcs and amass Zombies, you can end up with an Orc Zombie Army.

In the rare case that you control multiple Army creatures (perhaps because you played a creature with changeling) while you amass Zombies, you choose which of your Army creatures to put the +1/+1 counters on. If that creature isn't a Zombie, it becomes a Zombie in addition to its other types.