The Leviathans, the Chitauri's living warships, were invented for the Avengers movie and had to be designed from scratch. Avengers concept artist Phil Saunders has shared some of the early sketches for the Leviathans, including ones in which the ships had massive horns and this top one, which appears inspired by H.R. Giger's Alien xenomorph.

Saunders explains that originally the script called for two Chitauri craft, a "Jumbo" and a "Chariot." It wasn't clear at first if the Jumbo would be biological, mechanical, or both, but it was supposed to be whale or dragon-like from the beginning. Saunders began with a sort of mechanical parasitic worm, and went through many iterations of Leviathans before reaching the finished product. You can see some of those early designs as well as the Chariots below.


Avengers Alien Craft Round 1 and More Leviathans [Phil Saunders via Comic Book Movie]