Thunder was a rabbit from Efrafa, described as a powerful heavy veteran.
He took part in the Siege of Watership down and was only mentioned a few times. Woundwort relied on his advice and made him do some errands when they were digging out the Watership Down Warren. Thunder mentions he has found a deceased rabbit to General Woundwort. This is actually Fiver, who is not dead but unconscious. What happened to Thunder after the dog attack is unknown.