Sampras is Dougal's pet rabbit named after Pete Sampras (due to some bizarre perceived connection between tennis and rabbits that he doesn't explain).
Sampras appears to be a Dutch Rabbit breed and Dougal bought him from a travelling pet shop that wouldn't be visiting the island again until the spring, which he revealed to Ted after the house was overrun by the animals, due to the latter saying 'feck' to Bishop Brennan.