A DOTA 2 community member Cyborgmatt, who does detailed analysis for content released each week, recently spotted an unreleased courier model in the latest update. It was the Houndeye, one of the enemies from the original Half-Life. There is both a basic version and a flying version that can act as couriers in the game.

There have been several skins released for the default donkey courier in DOTA 2, with llamas, demons, robots, yaks and much more being made available.

Understandably, this would have people going “Oh it’s Half-Life 3!”. But it’s tenuous at best, wild speculation at worst. Some are theorizing that it is bonus digital content for those who pre-order Half-Life 3 if and when it is announced, which would make sense. As for when that’s happening, who knows?

The Houndeye was first seen in the original Half-Life and fought by using high frequency shock waves to damage and incapacitate players. The ground version of the model is seen with a barrel and Stout Shield.

(Source)