Lump by his mane

I was grooming Bobo yesterday and noticed a lump halfway up his neck at the base of his mane. It's ~1/2" long and hard. I can get it to move a little so I know it's on his skin but I can't see any reason for it to be there.

There doesn't seem to be any discomfort on his part. I played around with it this morning trying to determine if there was a bite of any kind or some kind of foreign object like a splinter but there I don't think so.

Any thoughts?

Wart like?

I don't think so. You would miss it if you weren't looking for it. The only way I found it was by feel. He had the grungies this winter but I can't see how it's related to that.