When I was around 11 years old, my dentist pointed out to me on the x-ray that there was some kind of a mass under my baby tooth in my lower jaw which was been preventing my first premolar tooth from growing out. He suspected it was an odontoma tumor and scheduled me to get it removed by an oral surgeon. I didn't get to keep my adult tooth because of the amount of jaw bone they had to cut out. I got to see the tumor they removed (which they said they were going to send to Baylor University for research). It looked like a bunch of tiny teeth teeth clumped together. They even had little roots.
I had an odontoma removed when I was around 20 years old. It was under one of my incisors and my adult tooth was impacted into the odontoma so they removed my odontoma, my permanent tooth and left my baby tooth - which today is still working fine. I'm now 41.
It took oral surgery to have it removed, and I was in the Army at the time so I don't know if it's a "dental" problem or a medical issue. Definitely benign or mine was at any rate.
I would suggest posting in the oral surgery forum.