The big, ugly villain our heroes unite to destroy is Zack Snyder's version of Doomsday, though it should be noted that the movie never explicitly calls him that. Lex says that the monster is Superman's "doomsday," but that's the first and last time the guy's official name is used. What's important to keep in mind is that Doomsday, for all his importance to comics fans, is still a relatively unknown character to most audiences.
As such, his inclusion here is a bit more of a wink and a nod to knowing comics readers, while less-informed moviegoers are getting their first glimpse at the bad guy who infamously killed Superman in the comics in the early '90s. But, as hinted at in the trailers, instead of arriving from space, the big-screen version has been engineered from the corpse of Zod, the Kryptonian maniac Superman killed at the end of Man of Steel, along with a bit of Lex Luthor's genetic material. In many other respects, though, he's still the same unstoppable killing machine.