These are bot fly larvae that were found in the stomach of multiple horses at our recent Scoping day. They don't cause a big issue in the horses stomach but worming with a boticide after the first frost each year will clear out this lot. We wait for the first frost as that is the end of the fly season.
Immature bot larvae do cause issues however as they tunnel their way through the gums in the mouth. Remember to comb the bott eggs off your horses legs in Summer to keep your horse happy and healthy!