There is some question about whether these creatures are real or legendary. The following is from old bestiaries that may not be especially trustworthy.
Belchers are reptilian creatures that range in size from large dogs to a small horse. The larger type, called Rock Belchers, are primarily subterranean creatures, dwelling in mountain caves. The smaller species, called Muckers, or Swamp Belchers, are partially aquatic and live in bogs or stagnant rivers.
Belchers have the unique ability to vomit a mixture of acidic spittle and poisonous gas. They use this primarily to tenderize their food, but can also use it as a projectile weapon against prey. In close quarters they also have a dangerous bite.