Most horses do it including mine, I just tend to chastise her when she does as it is annoying & if they do it all the time it can wear shoes down, especially on concrete etc.
None of mine over here do it (they are Quarter Horses, say no more Lol!!) but I had one in England who picked it up from a livery horse at my yard there. The livery horse's owners used to just let him do it - reasoning that if they ignored it, he would stop - did he hell!
Anyway as soon as my young horse decided this was fun, I told him off each and every single time .... along with the blooming livery horse when he did it. Both stopped fairly soon
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But then I never ever ignore equine behaviour, good OR bad.
I agree with Tia - my boy did it whilst going through his '5 year old stage', usually when tied up and he ddecided he'd been tied up long enough or could see me going to the feed room
I chastized him vocally every time, or gave him a quick poke if I was close enough and he soon stopped. I cannot abide this habit, pet hate of mine, so stamped on it asap. I particularly hate it when people allow their horses to do it whilst mounted.
QR
On a tangent, if your mare has just started doing this for no apparent reason, it could be an indicator of low grade pain, for example gastric ulcers and/or digestive problems.
Sometimes, in my experience, it can also be caused by a lack of fibre in the diet...typically when horses move from being in, to being out on lush spring grass.
Hope this helps
S
HMMMM I bought my youngster as a yearling & he has always done it, only at feed time or when he sees me coming. Ive tried everything, shouting ignoring, poking even a whip but he wont stop. I have now put a chain across his stable door so when he is in he doesnt paw & kick the door at the same time! Drives me and everyone else nuts! he also paws in the field when he is at the gate, i guess youthfull impatience but he is 3 now,ggggrrrrr dont no what else to do !