My 25 year old gelding has a big lump on the side of his neck, located right below the crest and maybe 4" behind the poll. It has been there for several years now and doesn't seem to bother him at all. However, I believe I am starting to feel a similar lump on the other side of his neck, directly across from it. My vet doesn't want to start messing with it as long as it is not bothering him, but I am more than a little curious as to what it could be. He has noticably aged this year; however he eats well and is otherwise in good health for his age.
I have a 34-year old Palomino mare who has had a small lump on her neck for the last 11 years. I asked the vet to look at it when it first appeared and he stated it was a cyst and he could remove it if it really bothered me, but that it didn't appear to be bothering the horse. I decided to leave it alone (I had hoped it would just go away) and it hasn't grown or changed in any way in 11 years. Needless to say, she is doing fine.