Well, I haven't heard from you in awhile, I see that you apparently have gotten past his aggression displays (and if that wasn't you, forgive me, I often get horses mixed up).
Let's start by addressing the color of the horse - green.
Loping or cantering requires the right amount of balance and rhtyhm ... rithum... rhythm... (long day), flow to go from trotting to loping. If the horse is off balance it is often easier for younger horses to dump the off balance load than it is to adjust to it. He is young isn't he?
Since this occurs the first time you lope (I assuming that is what you said anyway) I'd be inclined to think it's a balance problem.
Sometimes bucking in upward transitions is an indication you are proceeding too fast. Success will get you quicker than any mistake. We think just because a horse is doing so well at one thing that means it is ready to go on. Not always so.
Another thing that gets you is relying strictly on flat work to help your horse develop balance. The best place to help a horse build balance and become flexible and athletic is on the trail. This is also the best place to help the horse develop upward transitions. When the horse starts up a grade you can encourage it and allow it to pick up the canter on its own. As it picks its way over the ground it concentrates on balance and optimal movement.
When you learn how to adjust your gaits and speed on an ever-changing surface, changing on the flat is nothing.
The most important part about upward transitions with the green horse is to let the horse tell you when it's ready.
Now then, all this is assuming that the horse has no physical problems or discomfort that strikes at the initial weight transfer of the higher (faster) gait.
When a horse crow hops (bucks straight up) I usually suspect a thoracics area (withers to about the end of the rib cage) problem - saddle fit (high withers, sharp withers), spinal subluxation, or rib subluxation. If the horse bucks and kicks back, I look in the lumbar area and hips. If the horse surges forward and bucks I look for neck, shoulder, poll or dental problem.
To lessen the possibility it is physical I'd have a chiro look him over real good. If he is young, between 2-4, I'd give him some more time to just develop and lighten the demands on him. If he is 4 or more and green, I'd just wait for him to tell you when he's ready. You may think he's doing really well at walk-trot transitions and he may have a totally different idea.
Since this is your horse, you do not have to make any excuses for doing what you are doing. If someone is pressuring you to lope him and you're not sure he's ready just say, "I don't think he's ready," and let it go at that. If pressed, say, "I told you before, I don't think he's ready to lope and when I do think he's ready, we'll lope."
Don't let anyone show you how to "make" him lope. You will lose more than you gain.
Keep in touch and let me know how you're doing.
And then I got this:
See any similarity in what I suggested and what she discovered on her own? Sometimes backing off and going at just a little differently is all it takes. However, I still think there is a hampering issue of some sort. I'd still bring in a chiro and if that finds nothing, a saddle fitter and a CESMT (Certified Equine Sports Massage Therapist).
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