Hi Reggie_91, yes, I've commented on this before in respect of the Mounted Band. Tenor Horns (Eb) and/or Baritones (Bb). I'm guessing that they're the closest substitute for the sound of the French Horns. I should think that a French Horn would be almost impossible to play with one hand, especially while the horse is bouncing around. A Tenor Horn, and even a Baritone, is very much lighter than a Euphonium, and (normally) has only 3 valves so playing it with one hand is reasonably manageable.

Hi @@MikeM14-40 it only caught my eye there and never thought about it much. I can understand the switch of euphs to baritones to a certain extent although you could argue they could just use shoulder straps that the bases use. French horn to tennor horn however, while understandable from the logistical point of view of playing and riding that you have explained, is a different type of switch completely as they are very different to play and very seldom musicians play both. I do prefer tennors though but that's purely bias coming from a brass band background