Check out insurance companies before you buy a policy. Cheapest is rarely best. Some will have hidden exclusions and try their best to get out of paying and the last thing you need is a load of hassle when you are dealing with a sick or injured animal. Good luck witj your rescue pony.
I’ve got world horse welfare ponies and they insist you have third party insurance through them. As a rehomer it costs £30 a year and covers all my ponies. For a normal person it’s £52 a year I think.