My horse had something like that, they were a type of viral warts. Alarmingly they spread and started to go all over the inside of his back legs. The vet said they were fairly harmless and may well disappear on their own as my horse was young (about 3) or could be removed by scrapping off if necessary. I started him on large doses of vit e for other reasons and coincidentally they disappeared. They were quite regular round shape, flat and cream coloured. Can't remember the name now.