In Shakespeare: The World as a Stage, Bill Bryson talks about the many words Shakespeare invented. I thought I’d give it a go.
Uncanter: When a rider asks a horse to canter and the horse proceeds at an indeterminate gait that is neither a trot nor a canter nor a formal ASB gait. A horse may also uncanter when coming down from a canter to a trot. However, conservation of energy usually leads the horse to drop into a brisk trot instead.
Example: I lost the class because all I ever got in the first direction was an uncanter. [Show Report: SSF]
Note: During his epic fits, a Previous Horse would jump from a walk to a form of uncanter wherein each leg performed a different gait. This form of uncanter was extremely hard to ride due to the rider laughing her fool head off.
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New Off Topic post: Strangers on a Train.
Virtual Brush Box 
I would have loved to have seen Previous Horse’s version of uncanter. I’m laughing just trying to picture it!
We often calll that a Tranter (trot/rack/canter)! It’s always frustrating when a horse offers both leads… even worse when they offer all 4 leads =)
I think I’ve been successfully uncantering lately when I’ve been unsuccessfully trying to canter. So what exactly is a decanter? Deceleration of the canter?
I think I have decantered: “George locked all four legs and screeched to a stop.” https://rodneyssaga.com/2013/02/12/support-your-local-blogger/
That was definitely a decanter!