Yes it’s possible the pony was sold or otherwise rehomed along with another horse. She didn’t say sold, but she also didn’t say that she had moved them. The fence is solid and coherent and is not made out of random farm vehicles so maybe yes they are sold.
As far as hay weight. Honestly most batches of hay can have significant variations in flake weight and the big bales may have big flakes that you can subdivide or contrary that you absolutely cannot. If you have any weight concerns (I worry about my mare getting obese) you do need a reality check of weighing every so often. And if horse condition is going wrong either too fat or too thin the first thing you do is check weight of daily feed. it’s not rocket science
I would also say it’s not the first time I’ve heard a sketchy barn owner claim someone must be stealing hay because it’s disappearing too fast.
When the reality is that if you have 20 horses at an average 20 lbs of hay a day, you are feeding at least 400 lbs of hay per day which is about 4 three string bails or 8 two string bails.
That is a ton every five days. That’s six tons a month.