I've Played Every Borderlands Game, But One Has Completely Vanished From My Memory

I have played every Borderlands game ever made, including the spinoffs, and I love them all—or, at least, I thought I did. Earlier this week, I was browsing through the Borderlands Wiki page, doing research for another article, when I did a double-take. I was looking at a list of every Borderlands Vault Hunter, and six names stood out to me as being completely foreign. The playable characters were from Borderlands: The Pre-Sequel, a game I had no memory of ever playing.