Yes indeed, it's the root stock so get rid of it as close to the source as possible or you could lose the grafted part that makes fruit. As plants do not have anything like an immune system as we know it, a lot of related plants can be used as rootstocks. There are many plants related to apples, crab apples, medlars, pears & quinces...I don't know which you have there. The plant may have been chosen simply because it can be produced quickly, but sometimes certain plants are used as root stocks because they are resistant to a soil pathogen that the desired plant would be killed by. In the part of CA where I lived, there was something in the soil that attacked & killed English walnuts, so they would graft them onto black walnut rootstock.

Maybe it wants to be an African Thorn Tree?!?

I had considered the possibility of grafted root stock. I' had roses revert to root stock before but locust was the only tree I could think of with big thorns. That seemed like such a strange mix though. On @MojoDave s suggestion however, I looked up sour orange. I found no pictures of the thorns but multiple descriptions referred to the use of the root stock for grafting citrus.

The tree is 30 year old Bartlett pear, semi-dwarfed variety nursery stock, I suppose they could have grafted onto anything that would provide a solid foundation. On further research, i found that a common root stock for pears, chosen for dwarfing trait, is quince. Some varieties of quince do have thorns, although not as impressive as these.

I think grafting is the likely source of this growth. Doesn't really matter, I just though it was so odd when I found it.

Pomegranates have that as there natural winter over branching, if that is coming below the graft line, it could be the wild form of the tree.

Neat looking.

I have nothing valuable to offer but --- that looks MEAN!

I have black locust at my farm that looks like that.

Sour orange has thorns like that. It might be a sprout from the rootstock.

A suggestion - either draft in an editor then copy and paste or copy before pressing the "Submit Button".

Your alien looks dangerously alien. @Fernapple's suggestion sounds to be on the ball unless you have had those pesky Russians, Ukrainians, North Koreans and Chinese bombarding you with cosmic rays!

Are you sure that your tree is not grafted on to another rootstock of a different type. It can be in some cases a quite different but related species.