I've been thinking about this world that's been in my mind and one of the things in it are drake knights which are warriors who raised and ride domesticate breed of triceratops like the war elephants of our past. but I've been stumbling with the idea as it seems impossible for a normal person to ride a triceratops comfortably due to how wide the animals are. they are built like kegs and horses already are pretty wide so a animal that's even wider seems difficult to image someone riding plus the frill would seem to make to difficult to see over for a person which would be important to know what's in front of you. now I have an idea for a larger race to possibly ride them but right now I'm just curious if a normal person could ride a triceratops and if so how would the saddle have to be set to allow it to be possible. thank you for reading and have a good day.
First of all, trikes are approximately 2 meters wide and 2.9-3 meters tall. Horses are much smaller, meaning that a different approach to climb on to the trike must be used. I feel like people may have to either have a platform to get on, putting one leg on after another, then adjusting their position, or, they could just climb up the tail (not a good idea if the trike is aggressive or too large).
Like L.Dutch mentioned, I think that having a platform on the back of the trike would be extremely useful.
Once upon a time, outside the National Museum of Natural History in Washington, DC, there was a statue of a Triceratops (possibly named "Uncle Beasley"). Kids used to climb over the head and frill, up the back, and slide down the tail. It was possible to sit behind the frill with legs straddling the neck.
This only addresses when the mount is standing still. Whether sitting behind the frill would be useful while the animal moves about is unknown. It is possible that the motion of the neck and frill would make that spot uncomfortable really fast.
(Side item, I believe the statue is now at the National Zoo, and climbing on it hasn't been allowed in many years. Ah, those were the days.)
If the beasts were raised from birth like you say, setting aside the question of how to sit, the head frill problem could be solved by binding of some way. While the little trikes are young and the frill is still growing the frill could be bound in a manner similar to head binding was done with the Mayans or foot binding in some Japanese cultures. One could conceivably bind (mold?) the frill is such away to make room for the legs behind the frill as well as a 'dip' in the center to see over it.