Creature — Griffin
Flying Whenever a source an opponent controls deals damage to you, you may put that many +1/+1 counters on Retaliator Griffin.
The source of combat damage is the creature that dealt it.
If Retaliator Griffin is dealt lethal damage at the same time that you’re dealt damage by a source an opponent controls, Retaliator Griffin will be put into a graveyard before it would receive any counters. Its ability will still trigger, but it will do nothing when it resolves.
If an ability causes damage to be dealt, that ability will always identify the source of the damage. The ability itself is never the source. However, the source of the ability is often the source of the damage. For example, Deathbringer Thoctar’s ability says “Deathbringer Thoctar deals 1 damage to any target.”