Cheapest priced copy (The Lost Caverns of Ixalan Commander): $0.35 at Card Kingdom, near mint, their price list as of .
Bellowing Aegisaur 
Creature — Dinosaur · 3/5 · Uncommon · The List · #XLN-4
Enrage — Whenever this creature is dealt damage, put a +1/+1 counter on each other creature you control.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Lost Caverns of Ixalan Commander | 127 | Uncommon | $0.35 | — |
| The List | XLN-4 | Uncommon | $0.35 | — |
| Ixalan | 4 | Uncommon | $0.39 | $0.79 |
3 rulings from Wizards of the Coast
- 29 September 2017If another creature you control is dealt lethal damage at the same time that Bellowing Aegisaur is dealt damage, the other creature won't be saved by the +1/+1 counter that would have been put on it.
- 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.
- 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.