The Thunderbird, also known as the Spirit of Big Thunder, is the unseen main antagonist of the Disney Parks thrill ride Big Thunder Mountain Railroad, as well as a major unseen antagonist of Disneyland Paris' attraction Phantom Manor. It is an ancient elemental spirit that dwells within the mountain range and kills any who dares to disturb its territory.
The Thunderbird was a powerful ancient spirit regarded by the native Shoshone tribes as a guardian of the mountain range and wrathful entity. Their legends warned anyone of disturbing the mountains, lest they face the wrath of the Thunderbird with disasters and misfortune.
One day, the mountains were visited by Henry Ravenswood and his company, the Big Thunder Mining Company, who set up mines and settlements throughout the range. The natives warned Ravenswood of the danger of awakening the Spirit, but Ravenswood ignored them, leading to several tragedies.
Presumably the first of these disasters began in the settlement town of Rainbow Ridge (the Disneyland version of the attraction). The Thunderbird was awakened, and plagued the town with massive earthquakes that destroyed the town, killing dozens of miners and train engineers. Their ghosts would possess the trains and minecarts left over, causing the vehicles to speed around the wasteland of their own accord.
Thunder Mesa and Phantom Manor
The largest disaster was Thunder Mesa (the Disneyland Paris version of the ride), which was the town inhabited by the Big Thunder Mining Company's founder Henry Raveswood, his wife, and his daughter Mélanie. The Thunderbird attacked Thunder Mesa in 1860 with a massive earthquake that sent a large portion of the town into a canyon, killing every inhabitant of the town including the town mayor and all of the Ravenswood family except Mélanie. The ghosts of all of the deceased were either sent to possess the vehicles of the old railroad or be trapped in the canyon below Ravenswood's manor. Ravenswood himself would return as an evil spirit known as the Phantom, while the rest of the ghosts (as well as their skeletal remains) would linger on in the buried town.
The next town stuck by the Spirit's wrath was Tumbleweed (the Walt Disney World and Tokyo Disneyland versions of the attractions). In the 1880s, the Thunderbird cursed the land with severe drought, forcing many of the townsfolk to leave. A con artist salesman named Comulus Isobar came to town to sell "rain-making services". However, shortly after he was paid and was preparing to leave town, the Thunderbird struck Tumbleweed with a massive flash flood (tsunami in the Tokyo version), that washed him and the town away, killing him and the remaining townsfolk and leading to them possessing the leftover vehicles as well.
Though never actually seen, the Thunderbird is a violent, wrathful and ruthless guardian that is fiercely protective of its territory, and is not beyond killing everyone, both innocent and guilty, as revenge for disturbing its land. The Thunderbird cares nothing about the lives it takes, and has no qualms about tormenting the spirits of its victims by trapping them in objects or underground for all eternity.