Cat or dog (3)
I believe the answer is:
pet
I believe this is a double definition.
'cat' is the first definition.
(cat is an example of a pet)
'dog' is the second definition.
(dogs are often kept as pets)
(Other definitions for pet that I've seen before include "Cherished animal" , "Tamed domestic animal" , "Favourite, may be teacher's" , "Little darling" , "Darling; fit of sulks" .)