San Diego Zoo Global
San Diego Zoo: Hippotpotamus
This resource presents detailed information about the hippotpotamus, including several photos and an audio clip.
BBC
Bbc: Nature Wildfacts: Hippopotamus
This resource provides detailed information about hippopotamuses.
Other
The Ultimate Ungulate Page: Order Artiodactyla
An excellent site that provides a good description of Artiodactyla and then provides full descriptions of many of the different animals.