Looks like Titanfall's lore is rich of alien life in the form of assorted alien monsters, including a species of gigantic leviathans, as discovered by Red Robot's John Davidson in one of the latest ads:

NeoGaf user Amir0x also spotted some titanic skulls scattered on the Boneyard map. Judging from the tusks and the shape of the skulls themselves, it's most likely that they belong to the same species of monster:

Unfortunately according to Game Informer's Tim Turi, a conversation with Respawn revealed that the towering monsters won't actually enter the maps.

That's kind of a pity, considering that fighting those behemoths aboard our titans would have been insanely cool. Who knows...maybe there's still hope for the future. This is definitely a DLC I'd pay for, but if they're not on the menu for the first Titanfall I definitely hope we'll see more of them in a possible sequel.

That said, we can definitely confirm that the leviathans are a thing, as their concept art is featured prominently in the "Art of Titanfall" book that we just received for review purposes. We'll have to wait and see if Respawn will use them more prominently in future content, but I most certainly hope so.