we've been through similar scenario - re fencing too as your other post! We came to following conclusions after lots of thought / reading etc: Dogwood - yes we used, Hawthorn - again yes and accounts for 50% of what we have planted (about 1000 now total), Hornbeam - no idea but lots in mature woods surrounding us and some existing trees between fields and in old hedgeline and as far as we know ok with horses, Rosemary - no idea and not heard of in hedges either - be cautious with horses as evergreen (good around house as I believe mosquitos don't like), Field Maple - yes we used in first phase but as previous post from someone else yes it's from Acer family so looked at very carefully and seems to be ok with horses - make your own decision on that one as atypical myopathy from sycamore poisoning is horrible. (and despite what most people think sycamore is not native to UK and not good for native species imho
https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/british-trees/a-z-of-british-trees/sycamore/ ). But have avoided in phase 2 in case it puts off buyers way in the future if/when we sell up. Beech - don't know but probably ok? (check).
Trees: Silver birch, common lime, Beech, Willow. - not ones we checked out. Evergreens generally no go with horses including holly - something we looked at carefully as wanted a 40m run of it as a year round windbreak - obviously avoided. Blackthorn (sloe) someone mentioned yes great barrier and we have some self seeded and leaving - but avoided new for future again as some horse owners get very twitchy about the massive thorns! we also used common hazel, crab apple, dog rose, guelder rose, scotch rose. I don't know where you are but would suggest talking to local wholesale nursery who will also (hopefully) have knowledge about best local mix. we used
www.oakovernurseries.co.uk ...... good luck and hope this helps..........ps first time round we planted whips using spade - 2 inserts then lever up providing open 'slot'
https://www.youtube.com/watch?v=j2LGScCec6Q second time avoided - we are on clay which meant it wasn't as easy as it looks in this video (and back breaking) and in the summer when it dried out the 'slots' opened up and was concerned about exposed roots - however they made it through. Second time round (late March/April this year once waterlogging gone) used an auger (£12 amazon - more expensive ones out there but this one worked and description stated it would be replaced if broken - which it didnt
https://smile.amazon.co.uk/gp/product/B07VSF8RWT/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1 but looks like no longer available) on an 18v drill (£30 ebay). Probably took a little longer and on knees but much easier - long term hopefully better as broken up ground for roots to start but we'll see. Everything now on driplines too (other post) . good luck - hope this helps you and others doing same things, we spent a lot of time working things out ourselves as info we felt reliable was difficult to find........ oh, we also harvested hawthorn berries, rosehips, blackberries (brambles), sloes (blackthorn) and have pushed into ground in front of whips - hoping they will germinate providing a double row hedge eventually (bramble/sloe in selective areas!). Nature's way I guess but without a blob of bird poo! See what happens - has anyone else tried this? Again couldn't find anything...........