Rumpless is a term used for the mutation which causes the tail of an animal to be absent. Usage and examples can be found in Manx cats, Manx Rumpy, Belgian d`Everberg and Araucana chickens. ...
• (a.) Destitute of a rump.
Rump'less adjective Destitute of a rump. 
The Rumpless is a breed of bantam.
genetic trait in some chicken breeds where they have no tail