I've dug this thing up in my back garden in Waltham Cross a few days ago. Initially it looked like a bit of rotten wood but when I picked it up to toss it into the rubble I realised that it was too heavy to be wood. There are remains of what resembles bark but it is hard like stone.
to answer your questions firstly it could have been from one of the horses at the stables but it looks to old for that . What your seeing is patches of the original enamel that has worn away to reveal the ivory or dentine of the tooth . looking up the root end into the cavity of the tooth I can make out where the pulp of the teeth used to be