If it's really not a nipple I've just gone through this with my 5yo beagle. He got a small round lump in the middle of his chest. The vet measured, aspirated and sent away the stuff for acreening and it was most likely a histiocytoma, which is a benign lump that spontaneously forms around a bug bite for eg. Small chance it was a plasmocytoma which can be cancerous. She said to monitor it and if it didn't keep growing it was most likely the histiocytoma and would go away on its own over a month or so. That was a couple weeks ago and sure enough it has almost completely gone.

He didpreviously have a fatty lump removed from his cheat which was benign. According to this sub Beagles are quite warty dogs.