Wordnet 3.0
ADJECTIVE (1)
1. 
 living or growing on the exterior surface of an animal usually as a parasite; 
- Example: "an epizoic plant parasite"
The Collaborative International Dictionary of English v.0.48:
Epizoic \Ep`i*zo"ic\, a. (Zo["o]l.)
   Living upon the exterior of another animal; ectozoic; -- said
   of external parasites.
   [1913 Webster]
WordNet (r) 3.0 (2006):
epizoic
    adj 1: living or growing on the exterior surface of an animal
           usually as a parasite; "an epizoic plant parasite" [ant:
           endozoic, entozoan, entozoic]