Crups are a magical creature closely resembling the Muggle Jack Russel dog, originating in the southeast of England, and are clearly a wizard-bred dog. Physically they share additional features of the Jack Russel - they can have a wire-haired coat or a smooth one, they possess equally enthusiastic vitality - but however, their main corporeal difference is their tail, which is naturally shaped like the letter Y, vertically protruding from the end of their rump. This unique abnormality is often referred to as a "forked tail" and is typically required by the British wizarding law to be removed with a painless Severing Charm by the witches and wizards who own them, to avoid curious Muggle suspicions, so they can take their Crups out publicly without having to fear uncomfortable confrontations. A Crup's forked tail is commonly severed between six to eight weeks old, when they are cruppies [traditional term for young Crup pups]. A license from the Department for the Regulation and Control of Magical Creatures has to be procured and a test must be followed by the owner of the Crup to ensure the witch/wizard is capable of handling the creature.
One of the well known Crup breeders was Thomas Harkaway, the President of the Magical Congress of the United States of America in 1760 [however, this breeding abruptly ended when his Crups savagely attacked a group of Muggles/No Majs], and Celestina Warbeck, a famous singer, who bred rough-haired Crups.
The respected W.O.M.B.A.T deems that it may be possible for Crups to breed with ordinary dogs.