My neighbour's ponies are welcome into my fields to eat the grass off, saves me paying someone to top the grass for me, likewise my neighbour loves to see my sheep in her fields, though at the moment we've neither enough ponies 'nor sheep to keep the grass down.
I wouldn't let the ponies in in wet weather if they were shod, 'nor do I let them in if I have small lambs but otherwise they get on very well.