You need to take some of them to your vet and figure who they are. Then even though you just wormed you need to worm again using the drug designed to kill that species. Horses always have worms either eggs or adults, they are ground eaters that is where everyone else poops. So they are constantly injesting them. Most wormers attack only the adult worms, they do not kill and remove larvae and migrating stages. So those are the ones that you are seeing now. This is especially true for strongyles and ascarids.