Cheapest priced copy (Ixalan): $0.35 at Card Kingdom, near mint, their price list as of .
Snapping Sailback 
Creature — Dinosaur · 4/4 · Uncommon · The List · #XLN-208
Flash
Enrage — Whenever this creature is dealt damage, put a +1/+1 counter on it. (It must survive the damage to get the counter.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
2 rulings from Wizards of the Coast
- 19 January 2018If multiple sources deal damage to a creature with an enrage ability at the same time, most likely because multiple creatures blocked that creature, the enrage ability triggers only once.
- 19 January 2018If lethal damage is dealt to a creature with an enrage ability, that ability triggers. The creature with that enrage ability leaves the battlefield before that ability resolves, so it won’t be affected by the resolving ability.