Hector the Bulldog is a bulldog, the pet of Granny. Normally he hits Sylvester so he won't eat Tweety Bird. He is muscle-bound and has grey fur (except for A Street Cat Named Sylvester and Greedy For Tweety where his fur is yellow-ish.)
Hector first appeared in the 1945 short Peck Up Your Troubles paired with Sylvester and a woodpecker. His second appearance was in A Hare Grows in Manhattan. After that, Hector was a minor player in many Sylvester & Tweety cartoons, including 1954's Satan's Waitin' the devil. He also has a role in the series The Sylvester and Tweety Mysteries, where Sylvester is constantly outwitting him.
A dog called Butch, with a very similar appearance to Hector, appears in Birdy and the Beast (1944), predating Peck Up Your Troubles. Butch may be regarded as a prototype Hector.