By mid length do you mean kind of thigh length because if you do I've seen the long waterproof "drizabone" style coat in this shorter length at a local garden centre. They were about £50 max. You would need to put layers on underneath.
I have just bought a pony whose head is in between cob and full size - thinking of getting a bridle made for him. I have around £200 to spend bit wondered if anyone has any recommendations. Thanks