Traits

A rare mutation in which a leech sports a large frill of skin around its neck.

In the wild leeches use this frill to intimidate attackers, but they can also use it to express their emotions. A frill neck can just be plainly colored, but they often sport eye-like designs to add to the intimidation effect. 

Note: The frill can usually be raised and lowered at will, but can permanently be either or; and positioned anywhere on the neck itself.