I'd say it's just the wings of the atlas bone, unless you have some asymmetry?
less fine horses seem to have more flesh there so it doesn't protrude so much.
Right place - but very prominent. Might it be the Nuchal Ligament or Longissimus Capitis as well? Does it feel bony only when touched or is it a bit spongy too? With that slightly odd deformation I would wonder about longer term muscle stiffness or damage?