A swan nicknamed "Hannibal" after he killed 15 other birds and injured dozens more could be removed from his pond.
Hannibal lives with his mate, Mrs Hannibal, and their cygnet alone on Castle Pond by Pembroke Castle in west Wales.
When other swans stray into his territory he attacks them, said Maria Evans, a volunteer rescuer. She has applied for a licence to remove the family.
Ms Evans said: "He will corner them and duck them under until they drown or get them on to a ledge and smash their feet to smithereens with his beak."