He's big, mean, and green. Also pretty well-hidden.
If you don't want the boss to be spoiled for you, click away now.
The lumbering beast—who goes by the name Iwajira—apparently has a very upset tummy, which makes him vomit syrupy green all over players. The noxious stream has the side effect of being, um, fatal, so you should probably watch out for it.
You can find Iwajira by hopping your way into a hidden lava cavern in Serenity's Waste, Borderlands: The Pre-Sequel's first major zone, as demonstrated in the above video courtesy of GameFront. It's technically possible to stumble across him super early in the game (I nearly did when I was, like, level four while trying to make my way to a quest on the other side of the seemingly inaccessible lava cavern in question) but I wouldn't recommend it.
At level 30 and more or less invulnerable to attacks from players only a few levels below him, he's not exactly an easy one to bring down. Players are labeling him a raid boss, implying you might need a group to end his reign of painful puke-y-ness, but he's kinda low level by raid standards and lacks the "invincible" name scheme most (though not all) Borderlands raid bosses use. Perhaps there's a quest that sheds some more light on what exactly Iwajira is, but I never found it. I might dive back in and do some snooping soon, though.
For now, though, there he is. Go, admire him, tremble, etc. Borderlands games are typically loaded with easter eggs, so probably expect to hear about more of those soon.