HI Margery and welcome! I dont have horses here but have planted a lot of trees and living willow sculptures - and watched them being eaten at various stages by various animals! Willow in particular is nice and crunchy for quite a few years so I'm not sure a living shelter would survive unless you kept them away for a couple of years and even then I think that they might nibble the bark? For trees to establish to any kind of horse height shelter, Id say you would need to fence off for 3-5 years - I think the Forestry Commission recommend guarding for 5 years - so no 6 months wouldnt do it sorry

. I buy bare rooted whips from a couple of places Ashridge trees and Bernhards nursery - but you might have a local tree nursery - worth googling/yell.com . And Id recommend getting 2 year whips or larger (having them at 1m, rather than 50cm makes all the difference when it comes to competing against the grass and weed control ) and not bother with guarding unless you have rabbit probs, or staking.
I planted a few hundred trees this winter (bare rooted whips need to be planted in the dormant phase) and even though fenced, one of my sheep decided they looked delicious, found a way in and deleaved about 25 in about 24 hours!! Boo
The only possible exception that I have noticed surviving without fencing is hawthorn and blakthorn, but they are pretty stunted - having their juicy leaves nibbled off each spring - so not sure how many decades it would take for them to get to horse height. Good luck