StarCraft

Before StarCraft II was released, Blizzard released a DLC to the first game.

An excerpt from the article 15 facts about StarCraft

Brood War was released in late 1998 and supplemented the gameplay with a continuation of the storyline in three new campaigns, one for each of the space races.

New units were added to the game, including a medic in Terran, a lurker in Zerg, and a dark templar in Protoss.