Fact Monster: Animal Relationships (provides a good definition of mutualism, commensalism, parasitism)
Animal Odd Couples: National Geographic Kids (provides examples of mutualism)
The Science Spot: Use the search field on this page to search for your animal(s).
Symbiosis: A link to a Google book--use the Search in this book field on the left-side edge to look for mutualism, commensalism, or parasitism. DO NOT click on any links that say: "Buy this book". You will not be able to buy the book and will be wasting time that you could be spending learning about symbiotic relationships.
Also use NCWiseOWL, Elementary Zone, click on Kids Search, then search for mutualism, commensalism, or parasitism. If you are directed to SEE parasite, click the link.
You can also learn more about the relationship by searching for information about the organisms in the relationship.