Millennial bugs are insectoids which were assumed to have gone extinct one thousand years in the past. They had not gone extinct, however, but had merely entered a form of hibernation which lasts one thousand years at a time.
These bugs were the focus of a plot by Zurg, who obtained all the leftover millennial bug nests he could find in order to unleash them on the galaxy and destroy the Galactic Alliance (in the episode "Millennial Bugs"). His plan was foiled by Buzz Lightyear and Ozma Furbanna.