my husbands rottie had that on his foot and it turned out to be cancer, get your pet to the vet, and have a biopsy done
This kind of problem can have a variety of causes.
It may be a hot spot with a secondary yeast or bacterial infection, fleas or mites, or, it could be a benign or malignant dermal mass. It could even be caused by behavioral problems, and in this case it could be considered a bad habit, these behavior lesions are called acral lick dermatitis lesions. Utahmomma is referring to chemical irritations and this is also possible but usually affects more than one location.
Hot spots are usually due to food or environmental allergies. Food or environmental allergies can cause itching of the skin and can occur anywhere on the body.
However, the lesion could also have begun as a foreign body such as an awn, sticker, splinter, or wound, for example. These wounds can heal over but the inciting factor may still be there. Causing an abscess or infection.
The lesion could also be caused by ectoparasites, such as scabies mites, or fleas.
It could be caused by a benign skin neoplasm such as a histeocytoma, or malignant skin neoplasm such as a mast cell tumor as examples.
Lastly, Acral Lick dermatitis, is caused by stress, anxiety or other behavioral problem and must be treated with behavioral medications.
Whatever the cause, the dog perpetuates the problem by introducing bacterial from his mouth into the wound; this in turn causes a moist environment in which yeast thrives.
Your best bet is to have it examined by a vet who can perform the necessary diagnostics, so that it can be treated appropriately.
Do you live in an area of Washington that gets snow and do you walk him where there may be snowmelting chemicals or salt?