Lekgolo? Mkgalekogo? Something like that. They're a colony of worms that work together as a whole kind of like tubifex worms, only they're encased in a suit of armor and somehow they can shoot laz0r beams. They joined the Covenant because they wanted all teh cookies, they create finely-knit sweaters on weekdays, and they travel in pairs because they share a special bond. I think it's because they're one large colony divided into two.