Share 'Bolting horse'

I have an 18 hh warmblood who, whenever I try to take him away from the barn on his own will rear and spin around and run. I used to be able to ride him out on his own but ever since he discovered that cows exist, I get the feeling that that's what he's worried about. I'm a very experienced rider but have never had a horse that has made me feel so unsettled. I've had little horses do the same t...