A Jarvey is a magical beast that strongly resembles an overgrown ferret in its physical appearance. They are known to live below ground, likely in dens or burrows that they dig themselves. (Fantastic Beasts and Where to Find Them)
The most remarkable ability of the Jarvey is its capacity for human speech. However, its conversational skills are quite limited. A Jarvey's speech typically consists of a continuous stream of short and often rude phrases. It is not capable of holding a true, coherent conversation. (Fantastic Beasts and Where to Find Them) Jarveys are carnivores and primarily hunt gnomes, but their diet also includes voles, rats, and moles. This suggests they are skilled hunters and adept at burrowing to catch their prey. (Fantastic Beasts and Where to Find Them)
The specific history of the Jarvey is not widely documented. As a magical beast native to Great Britain and Ireland, it has long been known to the British wizarding community and has been officially classified by the Ministry of Magic's Department for the Regulation and Control of Magical Creatures. Its existence and basic characteristics were famously documented in Newt Scamander's definitive work, Fantastic Beasts and Where to Find Them.
The Jarvey is not mentioned at any point in the seven original Harry Potter novels. Its existence is part of the broader wizarding world established in supplementary materials, providing additional depth and detail to the magical fauna of the world.