Gastric acid
The highest concentration that gastric acid reaches in the stomach is 160 mM in the canaliculi. This is about 3 million times that of arterial blood, but almost exactly isotonic with other bodily fluids. The lowest pH of the secreted acid is 0.8, but the acid is diluted in the stomach lumen to a pH between 1 and 3.
Equine gastric ulcer syndrome
The digestive system of the horse evolved for its grazing lifestyle, where it would almost constantly eat small amounts of roughage throughout the day. Unlike carnivores, who produce stomach acid during meals, horses constantly secrete acid to help digest this source of grass, leading up to 9 gallons produced per day. Unchecked, the stomach acid can lower the pH to levels that will damage the gastric mucosa, leading to ulcers.