Hard Circular lump under bottom incisor

[Hoof_Prints]

Hello, looking for some opinions on what this could be ! Pictures to follow, need to charge up phone first. The pony is 5 years old and has a perfectly healthy normal mouth on one side, but on the other side the 403 incisor ( I think, been a while since I learnt tooth numbers!) is a lot darker as it has black/brown patches and directly beneath the tooth there is a circular lump about the size of a short fingernail. The lump is very hard and bony, and underneath the skin- the skin is of normal colour but the gum between the 403 and 402 is red and inflamed. The gum line behind the lump is also a bit raised. The lump causes no noticeable pain to press and doesn't move at all. Pony is very well in himself, bright and alert, eats perfectly well and weight is spot on.

It has got me stumped! Vet is coming Monday anyway for jab booster so will get him to have a look then. But Its playing on my mind so would like to ask for other ideas before Monday arrives

 

[Gloi]

It sounds like he could have had an injury in the past which has damaged that tooth, possibly broken it below the gum line so the part of tooth you can see is darker because that part is dead.

 

[Hoof_Prints]

That was an idea in had in mind, but we'll see tomorrow . Thanks for the reply
Now have photos...

the "normal" side

and the lumpy side

Anyone have any more ideas?

 

[Gloi]

It's hard to tell on the pics, are both the bottom corner teeth adult teeth? I'd still go with the idea that he's damaged the root in an old accident. I think that black tooth will need to come out at some point. Is the tooth next to it okay, that looks a funny colour in the pic too..