Didn't read the whole thread, sorry - it's long! But here's some I made up because there didn't seem to be any words for them out there :)
Flumping - a novice rider jumping in the most ungainly manner - often with many lost stirrups and bouncing.
Floopin' - riding for the sake of it, not doing much. Often seen with teenagers. (Not always!! But several I know of!)
Dimpsy - the time when it's not light or dark - between day and sunset. Riding out at this time tends to result in being lost in darkness.
Ego-bouncing - insisting that one's horse is hard to ride so that one can be considered a "good" rider.
Exploring - another term for "We got lost :/"
Bailing out - Jumping off a horse mid-gallop after a logical thought of "If I stay on, there is too high a likelihood of death.. I'm out!!"
These are mine, anyway - feel free to use, or scoff at :)