Dragon Egg

Run the first tier of the Old One's Army for this, not the last. Dark Mages show up in both, but the odds on the early waves are far kinder, and those waves clear in a few minutes without a full sentry loadout behind them. Grinding the hardest tier for a pet is the common mistake here.

Dark Mages are the enemy that fills the lane with skeletons, so kill the mage first and the rest of the wave gets a lot quieter. Sentries placed on the crystal side and a decent ranged weapon on the mage is all the run needs.

It hatches a Hoardagron, a small dragon that trails along behind you. Both the creature and its name come straight out of Dungeon Defenders 2, which is the game the whole event is a crossover with.