Hopalong Cassidy is a fictional cowboy hero, what was the name of his horse?

Hopalong Cassidy or Hop-along Cassidy is a fictional cowboy hero created in 1904 by the author Clarence E. Mulford, who wrote a series of popular short stories and many novels based on the character. He earned the nickname "Hop-along" after being shot in the leg.

As portrayed on the screen, white-haired Bill "Hopalong" Cassidy was usually clad strikingly in black (including his hat, an exception to the western film stereotype that only villains wore black hats). He was reserved and well spoken, with a sense of fair play. He was often called upon to intercede when dishonest characters took advantage of honest citizens. "Hoppy" and his white horse, Topper, usually traveled through the west with two companions—one young and trouble-prone with a weakness for damsels in distress, the other older, comically awkward and outspoken known as Red Connors and Windy Halladay.