The Gorebreather is a dragon mentioned three times in How to Fight a Dragon's Fury, as a violent dragon species fighting for Furious in the Dragon Rebellion.

"No, he would send Tongue-twisters, Gorebreathers, Brainpickers—all the most fearsome dragons in this dragon army"
―Furious' strategies
"She would send out a party of their most fearsome and pitiless dragons to destroy Hiccup before he got to Tomorrow: Hellsteethers, Tongue-twisters, Gorebreathers . . . just in case."
―Luna's thoughts
"Gorebreathers, Tongue-twisters, Hellsteethers, his brain told him helpful. Armed with poisonous gases, arm-twisting tongues, jaws that leapt out and get you . . ."
How to Fight a Dragon's Fury

The last passage tells us that Gorebreathers breathe toxic gas (Tongue-twisters twist limbs with their tongues and Hellsteethers have extendable jaws). No other description of abilities or appearance is given.