Short of finding an expert it might not be possible to ID it for sure at this point.
It appears to be a stump and the upper roots, but it's clearly well weathered, which is what you want. If I cut green wood, I age it at least one year outside before I even think of using it.. I'd guess this is a good bit older than a year. Chances of it still having any appreciable amount of sap or resins, which are the things that make some woods toxic to some degree, will be gone.
Given the shape, it does not look like any of the evergreens, with the possible exception, maybe, of cedar. But even cedar would be ok if it's well weathered enough.
I'd boil it if at all possible. If not, soak thoroughly, it may take some time to sink anyway, in either well salted water or some bleach and water. It will help leach out some of the tannins that darken water, and ensure no critters or such are alive in it.
Rinse it well and if you use bleach, make sure to allow the surface to dry or use a double dose of dechlorinator in the last rinse water. I've used bleach on many wood pieces and never had a problem so long as I used some extra dechlorinator before putting it a tank.