On my way to the Hyperion Info Stockade in olde Fyrestone Towne, a huge robot named Saturn drops out of the sky to stop me. It seems like at other points in the game when Jack has cooked up some particularly badass monster to slow me down, there is some exposition where he tells me how badly I am about to be beaten, or brags about how awesome he is, or there is some other explanation. But Saturn just shows up, says nothing, Jack says nothing, you just destroy him and move on. So is Saturn anything special (perhaps a movie/sci-fi reference I'm missing?) or just some random monster?
Saturn's just a random boss that always spawns there. He's a bit of a puzzle as I don't recall any other "boss out of freaking nowhere" moments in Borderlands, but he's almost a boss for the Info Stockade mission. He's optional though, you can just run past him if he doesn't kill you.
There's no story (unless I missed something in 2 full playthroughs) related to him. He does somewhat resemble Metal Gear Rex though from Metal Gear Solid (it's not the first MGS reference they've made either) but I have no idea what the name would refer to.
So yeah, he's just kinda there. He's "special" in that he's a boss but that's about it. No missions, but he respawns like other bosses.
Saturn was the originally the boss for that mission in Opportunity where you have to destroy all the statues of Jack, but Gearbox later decided he was far too powerful for a side quest of such little reward or importance.
So they replaced him with a badass constructor and added him to a later story mission to spice it up a bit, but didn't have any time left in the game's development to change or add some story about why he's there.
I think Saturn is supposed to be Jack's replacement for Wilhelm.