Sources : Ptarmigan

Pliny the Elder [1st century CE] (Natural History, Book 10, 68): ...and the particularly tasty willow-grouse [lagopus] belong to the Alps. The latter gets its name of 'hare-foot' [pedes leporino] from its feet which are tufted like a hare's, though the rest of it is bright white; it is the size of a pigeon. Outside that region it is not easy to keep it, as it does not grow tame in its habits and very quickly loses flesh. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.76): Lagepus is a bird, as Pliny says, which has feet like a hare's, and feathers similar to fur; whence he received its name. They have a white belly and are the size of pigeons. They do not feed easily outside, so when they find food they return to the caves, and thus live on plunder in secret. This bird's life never slows down, but when it is killed or dies a natural death, its body withers very quickly, and this, perhaps, because it has been constantly living in the withered earth. - [Badke translation/paraphrase]