Skin Issues on Horse

What are those quarter-sized, bumpy spots that some horses get?  I saw a horse once who had several of these spots that were a bit bumpy.  Unlike people and the need for acne treatments, I wondered if they were hives or some similar skin condition (like a human's eczema).

My horse has never had them.  I remember asking once a few years ago what they were to an owner of a horse who had them but I can't for the life of me remember what I was told.  Memory goes with age...or with a bad thyroid.