Today I was grooming Paddy's tail (it looks as pictured below about once every 5 years I should add - this was just a 'brush out the worst of the mud' deal). Elvis (the exmoor pony) came over to investigate. He often does this - he watches both the groom and the groomee(?) with great attention and will sometimes join in, grabbing bits of tail.
I could be wrong, but seems to me that he views this as play behaviour on the part of the human. If he were to pick up a bit of Paddy's tail and do stuff with it he'd definitely be initiating play. Interesting to think how often horses may interpret our actions in a totally differently way to us.