I insured my yearling with Pet Plan as they have categories of activity that mean you can save a bit of money if the equine isnt doing much
he started on the 'at rest, not working etc, doesn't leave the premises unless to go to vet' type category but was still covered for third party liability, that was about £11 a month
When he started going to a few in hand shows I moved him up to the next category and that was about £20
Insured mine from 7 months with KBIS, was about £20ish a month. Glad I did as she needed surgery cyst removing from her nose at 1yr and then she gashed her right hind open in the field at 18mo. They paid out both times no quibble, Cyst was about £1.5k and Leg ran just under the limit of £3k.
They got a bit happy with exclusions when renewal came around, so I moved to NFU.