Found a lump on my mare. When should I worry and call the vet?

Hello. It is always best to have all unusual lumps & bumps examined by your veterinarian at your earliest convenience. This lesion does not look particularly dangerous & actually looks more like some type of fly bite or chronic irritation, so you might just treat with a topical ointment for a week or so & see if it resolves. She is a young horse, & most cancers will develop at a mucocutaneous junction, such as near the eye, vulva or mouth. We rarely see a skin lesion turn dangerous in this particular region. But if you are at all concerned, have your vet out to take a look. Thanks for using Petco Pet Education Center, formerly Petcoach.