Neck mass?

[Ann in NH]

Looking for some guidance... Now that Cary (11yo Alpine wether) has finished shedding out, I see he has a huge mass on the underside of his neck, just above where it ties into his chest. It is not high up under his chin (goiter) and is below where his collar lies. It doesn't feel hot or tender; it's definitely firm (muscular, even). It's wider than his neck so that when viewed from above you can see it extends outward evenly on both sides.  It's about the size of a large honeydew melon.  What on earth am I seeing??  Sorry no photo - will try and snap one tomorrow.