Thunder is the stage name for the horse who is the official live animal mascot for the Denver Broncos football team. Thunder shares mascot duties with Miles, a human who wears a horse head mask atop a Broncos uniform.

Three purebred Arabians have held this role since 1993, all gray horses whose coats lightened with age until they turned completely white. Sharon Magness-Blake has owned all of them, and Ann Judge has been their rider since 1998 and trainer since 1999. As of 2022^[update], Thunder has appeared in four Super Bowls. The original Thunder performed in Super Bowl XXXII and Super Bowl XXXIII; Thunder III appeared in Super Bowl XLVIII and Super Bowl 50. Thunder III also made appearances in Times Square and on television morning news shows in New York City as part of the pre-game promotion for Super Bowl XLVIII.

Thunder's duties as mascot typically include leading the team onto the field at the start of every home game, and galloping down the length of the field whenever the team scores a touchdown or field goal. Thunder and his rider also interact with fans before the game; the horse is particularly popular with children, who are allowed to pet him. The horses who have served as Thunder need to remain calm in situations that would normally frighten most horses, such as being in football stadiums with thousands of cheering fans, exploding pyrotechnics, cheerleaders waving pom-poms, and other spectacles common to National Football League (NFL) games. He routinely appears in parades, makes hospital and school visits, and attends various other public functions. He has been flown on airplanes, ridden in elevators, and appeared indoors at press conferences and banquets.

The original Thunder, later named "Thunder Sr.", was described as bold and courageous. He was a stallion registered as JB Kobask, a former show horse, who was team mascot for the Broncos from 1993 until his retirement in 2004. He continued making community appearances until his death in 2009. Thunder Sr. was succeeded in 2004 by "Thunder II", an Arabian gelding registered as Winter Solstyce. He had been the personal pleasure riding horse of Magness-Blake. Judge described him as being somewhat timid when he first began his role as mascot but eventually grew into it. He retired from mascot duties in early 2014 but appeared in the 2016 Super Bowl 50 victory parade in downtown Denver following the Broncos win over the Carolina Panthers. "Thunder III", a gelding registered as Me N Myshadow, was the understudy to Thunder II, and trained specifically for mascot duties beginning at age three when he was started under saddle. He began performing at preseason games in 2013. Although Thunder II was still active as team mascot during the 2013–14 season, Thunder III was sent to Super Bowl XLVIII because he was younger and better able to handle air travel. He is described as laid-back, preferring to doze off during games when not performing. Thunder III ran one last time during the 2020 season. Thunder IV, registered as Phantom JD, is the current mascot and began making appearances in the 2021 season.

The original "Thunder", JB Kobask, was loaned to the Denver Broncos by Sharon Magness-Blake of Magness Arabians after she received a phone call from team officials asking if she had a "white" horse. Thunder's debut as team mascot was on September 12, 1993, during the Broncos' victory over the San Diego Chargers. Angela Moore, his rider at the time, galloped him down the field after each Broncos touchdown. Thunder's mascot duties were expanded to include delivering the game ball to the referees at the start of each home game. Spectators were allowed to interact with Thunder and pet him prior to the game. Magness said, "Thunder is friendly, and Broncos fans think it's good luck for the opposing team to pet our mascot". He was particularly popular with children, appeared on T-shirts, and was referred to as the "second most popular Bronco" after former quarterback John Elway. Thunder appeared at Elway's 1999 retirement gala. When team uniforms were redesigned in 1997, Thunder was the inspiration to incorporate a horse-head profile as part of the logo on the team's helmets. During a February 4, 1997 press conference introducing the new logo, the team president and the art director for Nike, creators of the new design, described it as "a powerful horse with a fiery eye and mane."

The original Thunder, also known as Thunder Sr., was retired in 2004 after serving 11 years as the team mascot. Magness-Blake replaced him with Thunder II, a 1994 Arabian gelding registered as Winter Solstyce. He was Magness-Blake's personal pleasure riding horse prior to taking on his new role as team mascot. As Thunder II grew older, Magness-Blake anticipated the need for a replacement and added a third gray horse to the team. He became known as Thunder III, a 2000 Arabian gelding registered as Me N Myshadow. Thunder III initially served as the understudy for Thunder II, and was trained specifically for the role of team mascot from his start under saddle.

Thunder leads the team onto the field at the beginning of each home game, and typically gallops from one end of the field to the other whenever the team makes a touchdown. He may also make a run after a field goal, but not a safety. Each time he makes a run, a four-person crew runs down the sidelines to the end zone to escort Thunder back to his sideline position. Extra handlers ensure the safety of the horse and surrounding people. Thunder's helpers also remove any manure that he might drop while on the field. As well as his support crew, Thunder has a bodyguard who travels to the games. Thunder shares mascot duties with Miles, a human who wears a horse head mask atop a Broncos uniform.

The horses who have served as Thunder have been trained to remain calm in situations that would normally cause a horse to respond with a fight-or-flight response, such as the flashing explosions of a pyrotechnic display, or items suddenly landing on the field including skydivers with parachutes and various objects thrown by spectators, or loud music and the sounds associated with tens of thousands of cheering fans at Mile High. Ann Judge has explained that because so many unexpected things can happen, it is important that the people working with him remain calm; it is trust in his rider and handlers, not just desensitization, that helps the horse remain steady: "You want them to look to you for the appropriate response and for confidence and faith so that the flight response doesn’t get initiated." She describes the process by saying, "if something is fearful to him, as long as he looks to me, and I tell him he’s OK, he’s OK." The only frightening stimuli that was an ongoing problem for both Thunder Sr. and Thunder II was when the crowd would perform the wave; Thunder Sr. would stand still, but tremble. Thunder II would become tense, requiring ground handlers and his rider to help steady him. Thunder III is the first of the mascots to tolerate wearing earplugs, which help protect his hearing from loud noises during games.