The most vulnerable horses to being stolen are
coloureds
mares
up to about 15,2
minatures/shetlands
Coloureds because they are fashionable so worth more £££ and will mix in easily with the rest of the coloured herd the thieves have. Mares for breeding with. Smallish so that they can be hidden more easily. Shetlands/minatures because they are the easiest to hide even in the back of the transit.
This comes straight from the mouths of our former nearish neighbours at the local travellers site at our old place. Who didnt steal our horses, cos our yard owner had 4 Rotties and cos according to us when they got nosy all our horses were geldings or mares with terrible gynaecological problems meaning they were no good for breeding :-)))). But they did steal electric fencing, rugs (even postcoded ones, happily displaying the postcodes on their own horses a few days later....:-O), the CCTV camera the council put up to try and stop the flytipping etc etc.....
Please make sure all mares coloureds and littluns are freezemarked and microchipped. If you have a small coloured mare I would let it live in your garage!