Large circle on my dogs lip!!!

[RXkp60]

Hi im just wondering if anyone knows what this might be...??
2 Days ago on the lower part of my dogs upper lip i thought there may have been a reddish circle but couldn't decide if it was or wasn't there.
Next day there is definitely a circle there which is now almost black in colour. it doesn't look scabby or anything. and possibly only slightly swollen.
Now it is very dark and very pronounced.
Doesn't seem to be giving him any pain.
There's no hair missing, inside the circle is just normal like the rest of his lip.

[RXkp60]

Thank you i will do some research into this histiocytoma. I did have his usual veterinarian examine him, who had no idea what it was. She prescribed Prednisone and also Neocort creme.
I took the Pred. straight back as would never give that to my dog (Especially when she couldn't give a proper diagnosis)
I have been applying the Neocort but not sure if it is helping or not - however it does appear to look better.
I was starting to think it may be some kind of insect or spider bite.

[Cliff Redford, DVM]

It sounds like a common viral induced mass called a histiocytoma. Either way the simple rule is every lump or mass should be inspected by a veterinarian asap. Early detection is the key to maximizing prognosis.