"The Green Buffalo" is a short story by Harry Turtledove. It was first published in The Ultimate Dinosaur, edited by Byron Preiss & Robert Silverberg, Bantam Spectra, 1992 and reprinted in Counting Up, Counting Down, Del Rey, 2002.
The story is narrated by Joe, the owner of a store in Lusk, Wyoming in 1891. He is hired by paleontologist John Hatcher as part of a hunting party for his expedition. While on the trail, the hunters wander through a ripple in time, and are briefly transported into Wyoming's prehistory. After backtracking, they return to 1891, and track a herd of buffalo. They discover a Torosaurus among the herd. Joe and his fellow hunters don't realize what the creature is, and assume that it is some sort of diseased green buffalo. The hunters chase and kill the dinosaur, butcher it, and bring the meat back to Hatcher at the dig site. Turtledove's trademark irony arrives at the end, when Hatcher offers to show Joe a newspaper cartoon featuring several dinosaurs, including Torosaurus. Joe refuses, and so neither man learns that their supper is not a diseased buffalo, but a dinosaur.