Giant ant

Giant ants are hostile creatures commonly found in the wasteland, including the American Southwest, the Capital Wasteland, the Mojave Wasteland, the Commonwealth and Appalachia.

Background
Ants are mutated insects of the genus Camponotus. Mackay from Vault 101 theorized that the creatures mutated due to extended exposure to radiation. 

Behind the scenes

 * Giant albino ants were planned for inclusion in Van Buren.
 * Ants are mentioned in the Fallout Bible.
 * According to Mark Lampert, the sounds for the giant ants in Fallout 3 were created using various plastic wrappers, clear wrap and liquids.