Little crusties belly

[ShetlandponyMama]

I can’t for the life of me remember what’s wrong with my little Shetland.
Clipped her belly today (winter here).
she had wee mats of crusties and when picked off it was yellow. The area is 2 or so inches directly under flank towards belly. 2-3 inches forward from udder. It’s swollen (fits in palm of hand) and very itchy. She was very grateful to have it cleared away.
didn’t put anything on. Only on one side. No bald patches.
lice?
sweat rash?

She’s not on long grass

thanks

 

[DressageCob]

My horse had something which I would describe in a similar way, except it was on him rump. Turned out to be a staph infection.

 

[Auslander]

My horse gets bitten by flies in that area, and it can get swollen, itchy and oozy.