I’ve got a 5 year old cob gelding who has recently (within the last month) been introduced to a new gelding companion and moved into a new field. They share the field with 2 other geldings who go out at night while mine is out during the day so there’s some overlap and they do spend a couple of hours together. They’re all young - 6 and under. Mine has started mounting 2 of the others - he’ll do it as soon as they get turned out or if the others start playing he’ll try and jump on the 4th one. There doesn’t seem to be much hierarchy out there at the moment. There’s nothing in his passport about when he was castrated but he definitely doesn’t have dangly bits. Their paddock isn’t particularly big so sooner or later there won’t be enough grass out there to distract them.
He was turned out with a mare previously (just the two of them) and I didn’t see any of this behaviour - he was in charge. She was quite mareish and was squirting at the mares in the next field so I don’t think it’s a hormonal thing because surely he’d have reacted to that? Generally he’s not that bothered if other horses are around or not - doesn’t get anxious or call and doesn’t fret if he’s alone in the stable or the field.
I’m thinking of getting the vet out to get him blood tested to see if he’s a rig and looking into supplements but I worry if I separate him it’ll be more difficult to integrate him back in later. To make things more difficult a new young gelding will be arriving in about a month and will be going in with them.