If you have fully let the pony go and it’s like a full loan it is really up to the level of the pony and what discipline. I’ve seen 14.2hh ponies going round the FEI circuits leased out to the kids for 1000’s because parents will pay it.
If you were to allow the leasee to come out to the barn to ride or whatever, whenever she/he felt like it, I would charge $800 per month. This is in USD. If it were a yearly lease and offsite; for a well trained horse/pony i would go with $5,000