Reading it with an editorial eye (so comments before completion):
but in the first line is clunky. Better as two sentences.
The leather/sweat of [Milo's] horse makes sense. The gress isn't of the horse. As such, the sentence is illogical. Also, the multi-slashed noun construct might be fine in a business or technical setting, but doesn't read well in proce.
Repetition of burst between second and third paragraphs.
You are head-hopping in the third paragraph, giving us the boar's thoughts. So, unless the lead-up has Milo in the boar's head, and he's still there (so could/should push back to dampen its aggression), letting us the boar is maddened is head-hopping.
Para 4 doesn't make physical sense, unless there's a cliff behind the boar that you haven't told us about. Or were the beaters on the far side, pushing the boar towards the party? (That may have been covered before the extract.)
Those last two issues repeat a few more times in the remainder of the piece. (It can help with something like this to map out exactly where everyone is every second or two, to ensure realistic movements. And keep in mind that when people are dazed, it lasts far longer than you would think. So the rescued beater will crumble, shake his head, scramble to his feet, then start clawing at a tree, by which time Milo will be long gone onto his next actions.)
And what Maggie says about the adjectives and adverbs.
There's a lot of scope for tightening, but it has potential.