The Snallygaster is a dangerous, dragon-like creature that inhabits North America (Pm).
Isolt Sayre and William the Pukwudgie fought a Snallygaster at some point (Pm).
Snallygaster heartstring can be used as a wand core (Pm).
This delightfully fun name is not an invention of Rowling's, but developed from the name German immigrants gave to a monster that supposedly attacked their settlements in the 1700s. They called it "Schneller Geist," which means "quick ghost" (source: Encyclopedia Mythica).
The Snallygaster is a mythical winged monster that supposedly lives in and around Frederick County, Maryland. There are varying accounts of what the Snallygaster looks like, but it is often given avian attributes and, most commonly, said to have dragon-like characteristics such as scales. There is no similar creature in Native American myth or folklore from the region, and it is believed that the concept of the Snallygaster developed from the dragonlore early settlers brought with them to America (source: Encyclopedia Mythica).
From the Web
The Snallygaster, from Ghosts and Legends of Frederick County by Timothy L. Cannon and Nancy F. Whitmore