The flashback in the movie was originally going to focus on Manny's conflict with his first family. However, Queen Latifah pitched the idea of the flashback in the film to center more on Ellie's origins.
The two sea reptiles that chase the Herd throughout the film are named Cretaceous and Maelstrom: Cretaceous, an ichthyosaur, is the purple crocodilian, while Maelstrom, the stout green reptile, is a pliosaur. There was initially going to be a third predator, who appeared in some early designs as a small axolotl-like creature, but it was cut out. The back story for the villains is that they were frozen millions of years ago and kept in suspended movement.
As an additional marketing ploy a special "anti-cell" spot was created with Sid complaining to the audience about a ringing cellular phone. The same was done for Brother Bear (2003), Robots (2005) and I, Robot (2004).
Surprisingly, animals similar to the horned beavers did exist. Ceratogaulus was a horned rodent, with the horns likely used for defence. However, it was not closely related to beavers, belonging to a separate and now-extinct family, and lacked the large, flat tail which it has in the movie. It more closely resembled a marmot or gopher.
The flightless bird mom carrying her nest might be Dromornis aka: "The Demon Duck Of Doom". It was from Australia & distantly related to ducks, but had a head more like a parrot & had a body like a buffed up ostrich.