Ruthless

Ruthless is R&D slang term used for “When ~ enters the battlefield, put ... -1/-1 counters on target creature you control.”

The black and green mechanic was featured in Amonkhet. In design it was actually an ability worded as Ruthless but R&D ultimately didn't feel they had enough cards to give it an ability word. It was referenced in the name of Ruthless Sniper.

There is often a second ability on the cards with Ruthless, most of the times interacting with -1/-1 counters.

Cards with Ruthless

 * Baleful Ammit
 * Crocodile of the Crossing
 * Channeler Initiate
 * Decimator Beetle
 * Defiant Greatmaw
 * Exemplar of Strength
 * Lethal Sting
 * Manticore of the Gauntlet</c>
 * Ornery Kudu</c>
 * Plague Belcher</c>
 * Soulstinger</c>