Abigail Bullion, sometimes nicknamed Abby, is or was a human woman.
The daughter of wealthy Barnabas T. Bullion, owner of the Big Thunder Railroad Company and of a mine in Big Thunder Mountain, Abigail Bullion was, as a young woman, much more adventurous and reckless than would be considered "proper", enjoying nothing more than trick-riding with her horse Jaggers. In 1878, upon coming back from her studies abroad to Rainbow Ridge to live with her father, she wasted no time in embroiling herself in various adventures, meeting and falling in love with Robin Hood-style outlaw Jason Chandler, Jr in the process, while also discovering the truth about the Thunderbird and helping her father see the light.
Behind the scenes
Abigail Bullion is the main character of the Big Thunder Mountain Railroad comic printed in 2015 as part of the Disney Kingdoms event. She did not appear in any version of the original ride, but is believed to have drawn some inspiration from some depictions of Melanie Ravenswood in life, being the attractive and willful daughter of the local wealthy but questionably-moral mining baron.
It is unknown if she is still alive in the present day; while it would be enormously unlikely, if not downright impossible, in the real world, one must not forget that Scrooge McDuck (very much alive in the Prime Universe's present day) is only a few years younger than Abigail.