I’ve had this mare for 8 years. I was giving a BLM mustang as a gelding. It would be a year June 5th 2021. Now I’m not sure if he was actually gelding or not, (he was abused so he was real finicky around his hindquarters I could never get close to look)
I would say fat, as your picture of your mares teats and what you think looks like bagging up isn't milk, it's swollen glands from being overweight. Mine looked exactly the same underneath until she ditched 40kgs at rehab a few weeks back and then it went down.
Also no matter how close to a back end you can get horse balls are huge so are pretty easy to spot if your gelding was still entire with him just moving about in field.