Usborrak is basically a colony of small arthropod-like Bydo that operates similarly to Shell. In addition to the blue core which wipes out all of them, yellow Usborrak are inoffensive and red ones shoot Bydo Shots. As a supposition, it's possible that this group of semi-organic Bydo was remodeled after the Shell defense system or vice versa. So far, they only appear in Decaying Erosion Cave.
Gallery
Concept artwork of the Usborrak. Note the differing body segments yet are still part of the same body.