We have a mixed herd of 17 horses. One of them is a new forest gelding and I noticed this morning that he has got 2 fillies with him and herds them about. If any other horse goes anywhere near he runs at them and spins round with both back feet. I would worry about this behaviour as he is also very nippy and rude in the stable. What do you guys think? Rig?
Personally I would say not, unless he is actually mounting them. My elderly gelding (nearly 30) has been in a herd with our girls for almost 15 years and he was very protective over them, but never attempted to mount them. He would roar at any geldings in neighbouring fields and would herd the girls around if he thought another 'man' was after them. Now he is older he has welcomed a younger gelding into the herd and I think he is quite relieved that he no longer has to play the protective role!
It is instinct for them to behave this way while they are living as a herd. However in your case in a more mixed herd I expect he is trying to band his own little 'hareem' together! In the wild, young colts generally cannot take over their herd and will roam together as a bachelor group and pick up young mares from other herds to make their own!
The last yard I was on had a very riggy gelding, he was out with two mares and would cover them both. If one was brought in, he would just walk through the fence to get to them. (He was a big Suffolk Punch cross thing). He was very bossy with his owner and if she brought him in away from them she couldnt do a thing with him - he wouldnt leave the yard so she couldnt ride him. He was a bit of a pain as he would not let the owner of the mares catch her own horses as he wouldnt let them go.
Mine live in a heard of 22 - [mixed mares and geldings, there is a welsh mountain pony 12.2hh who regularly rounds up 3 mares to keep to himself - sometimes they play along and when they get fed up they just wander off. There is also a 14hh pony who will on occasion mount a mare - if she is willing? and you would believe to see him he could do the job (but he can't) so I would say this is normal behaviour in a mixed heard.