Stampeding Elk Herd

Formidable — Whenever this creature attacks, if creatures you control have total power 8 or greater, creatures you control gain trample until end of turn.

Pioneer Masters

The Atarka use them not just for food, but also to clear away snow, trees, and enemy forces.

Dragons of Tarkir

Formidable — Whenever Stampeding Elk Herd attacks, if creatures you control have total power 8 or greater, creatures you control gain trample until end of turn.

If you control a creature with power less than 0, use its actual power when calculating the total power of creatures you control. For example, if you control three creatures with powers 4, 5, and -2, the total power of creatures you control is 7.

Other formidable abilities are triggered abilities with an “intervening ‘if’” clause. Such abilities check the total power of creatures you control twice: once at the appropriate time to see if the ability will trigger, and again as the ability tries to resolve. If, at that time, the total power of creatures you control is no longer 8 or greater, the ability will have no effect.

Some formidable abilities are activated abilities that require creatures you control to have total power 8 or greater. Once you activate these abilities, it doesn’t matter what happens to the total power of creatures you control.