Tenontosaurus was discovered by, surprise!, an American Museum of Natural History expedition in Montana in 1903. Barnum Brown went on to name the animal Tenantosaurus, "sinew lizard". 18 specimens were unearthed and in the 1960's John Ostrom dug out 40 more examples. His description was the first, and only, and his name, Tenontosaurus, is what stuck. It's really just a change in spelling from Barnum Brown's Tenantosaurus.