A horse that reaches well up into the girth area when cantering at liberty will have a comfortable canter. This same horse should have a good overtrack at the walk.
Natural rhythm and a strong hind leg. A horse that is naturally relaxed and can carry him or herself doesn’t hurt either.
My horse has a super comfy canter, even when he’s going around like a coked out giraffe (which happens more than I’d care to admit…love it when he reminds me that he was once a (sometimes) Very Fast Racehorse). His trot’s decently comfortable and he’s being a good teacher for me as I attempt to learn how to sit lengthenings properly. He’s not very extravagant and he’s pretty responsive and trustworthy. His “bucks” are actually kind of amusing to watch – bless his little cotton socks, I don’t think my butt’s ever left the saddle during one of them.
