The Crill, or Krill, is the name given to a lizard species of the Draco system. The name originally was the name of one of the "ambassadors" that came to stay on Earth after the US Governement had its first official contact with them. Later on, the name got used to indicate the species "ambassador Crill" belonged to.
The late Bill Cooper, in the Krill Report, claimed that O.H. Crill (or Krill - both spellings are used), was one of the sources of the Yellow Book. Other researchers refer to an exchange ambassador known as 'J-Rod' as (one of) the source(s) of the Yellow Book.