From PBS's Horatio's Drive (documentary you can watch on netflix!):
As early in the trip as Sacramento (and for reasons unexplained), Jackson had been looking for a small dog to accompany him on his attempt to cross the continent. In Idaho, he finally got his chance -- thanks to the kind of misadventure that had now become almost routine. They had pulled out of Caldwell early on the morning of June 12, but after driving a few miles Jackson realized he had left his coat at the hotel. "On our way back," he wrote Bertha, "we were stopped by a man and asked if I didn't want a dog for a mascot. As I had been trying to steal one we were glad to get him so accepted the present (consideration $15.00). So Bud is now with us."
Bud was a young, light-colored bulldog, and whatever the rationale for adding this third member to the expedition, he almost immediately began attracting as much attention as the Vermont. Some newspapers would report that Jackson had rescued Bud from a savage dog fight; others wrote that he was a lonesome stray who had chased after the car for two miles before being taken on board.
"Bud soon became an enthusiast for motoring," Jackson bragged, especially after his masters put a pair of their goggles on him to keep the stinging, alkali dust out of his eyes. Riding in front, Bud learned to watch the road ahead as intently as Crocker and Jackson, bracing himself for every bump and turn -- and becoming, his owner said, "the one member of [our] trio who used no profanity on the entire trip."
Bud apparently lived a full dog's life after the cross-country trip, content to guard the Jackson home in Vermont and take short automobile trips around Burlington with his master. Following Bud's death, the Jacksons always kept at least one dog in their house, although none ever became as celebrated as the begoggled-bull pup who had crossed the continent.
http://www.pbs.org/horatio/wheel/ x-posted to
bygone_dogs