Teacups are not a species of ray and the name is used to describe rays as CFC said and also as a common name for a particular size, of small ray (these could be Retics, Humerosas, or Histrix...). Why this is called 'Teacup' I have no idea as the rays are normally 8"+ not 3"!
EDIT: Is he in a 30 gallon tank!?