Bulldog

wiktionary

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bull +‎  dog, in reference to  bullbaiting. 

etymonline

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bulldog (n.)

also bull-dog, "small, strong, muscular kind of dog noted for courage and ferocity," c. 1500, from bull (n.1) + dog (n.). So called perhaps from the shape, perhaps because they originally were used for baiting bulls. In U.S. newspaper slang, the bulldog edition (1910) was the earliest run of a daily paper.