https://www.gamespot.com/articles/free-diablo-3-battlecruiser-pet-is-now-available/1100-6457221/

Diablo 3 has gotten its own StarCraft-themed goodie in celebration of the space strategy's 20th anniversary. You can now claim a pet called Dominion's Revenge, a Battlecruiser given a very red, black, and gold paint-job. To receive the special pet, just log in to the game and load up a character, and you'll automatically receive an achievement that grants the pet as a reward.

The Diablo love is just one of several in-game promotional items across Blizzard's games to ring in StarCraft's 20th anniversary coming on March 31. Others include portraits and decals in StarCraft: Remastered and StarCraft 2, a Kerrigan skin for Widowmaker in Overwatch, a special Tavern Brawl in Hearthstone on March 21, anniversary portraits in Heroes of the Storm on March 27, and special StarCraft pets in World of Warcraft. Presumably the Diablo 3 pet is only available for a limited time, as many of these goodies expire around the end of the month.