sweaty, hard area on 33 yo mare's haunch?

I first noticed an area on my 33 yo mare's haunch, left side only, near her tail, which was greying out and was always sweaty. Day, night, cool weather, whatever, it was always sweaty. While I don't think its gotten much larger (~6" diameter), it now seems to be firmer, almost hard to the touch. No one at my barn has any idea what it could be- doesn't seem to be painful, or affect her movement in any way. As she is old, I always worry about the usual- colic, cancer, etc.

You sure it's sweat and not somekind of drainage? As, in maybe an abscess. Maybe scratched her hiney on a tree and got a splinter that got infected?

Excellent answer. I'd give that area a surgical clip & see what is going on.