Acorn was one of the original rabbits that left the Sandleford Warren with Hazel and Fiver. He was good friends with Speedwell, Buckthorn, and Hawkbit. Acorn is mentioned as being one of the most skilled rabbits when it comes to gathering bugs for the wounded Kehaar, and is unflaggingly loyal to Hazel in the battle of the warren against General Woundwort.
In Tales from Watership Down he dies due to wintertime hardship in the Watership Down Warren, and is as such the only one of Hazel's original crew to actually die over the course of the two books (not counting Hazel‘s own death in the future).
He is one of the rabbits from the book to not appear in any of the adaptations, along with Buckthorn, Bluebell, and Speedwell.