Symbols of the Founders - Animals and Elements

House	Element	Source	Animal	Source
Gryffindor	Fire	See Elements Quote Below	Lion	Book 4, Page 237
Slytherin	Water		Snake
Ravenclaw	Air		Eagle
Hufflepuff	Earth		Badger

The Elements Quote:
JKR: Probably. I hear you. It is the tradition to have four houses, but in this case, I wanted them to correspond roughly to the four elements. So Gryffindor is fire, Ravenclaw is air, Hufflepuff is earth, and Slytherin is water, hence the fact that their common room is under the lake. So again, it was this idea of harmony and balance, that you had four necessary components and by integrating them you would make a very strong place. But they remain fragmented, as we know.
Source: Accio-Quote

Links:

Table at HP-Lexicon includes the Ghost, Animal, Colors and Head of each House.

In the House Points page HP-Lexicon lists the Jewels of Gryffindor (Rubies), Slytherin (Emeralds) and Ravenclaw (Sapphires).