I have a small farm and just 4 weeks ago I brought home 2 OTTB’s. One I had just gotten off the track, he is 4 and he is so sweet and gentle. The other one is 7 and he had been living with my friend and her OTTB’s for the past 3 years. She has around 8 other horses but he was always kept in the paddock with just 1 or 2 friends. She kept them in a stall next to each other for a day before travel for them to meet and get to know each other. She trailered them down here for me (around a 12 hour drive) and they traveled perfectly. When she got here we turned them out and everything went smoothly. For a few days. Around day 4 I started seeing some aggression from the 7 year old to the 4 year old. I thought Norma hers things. He’s just establishing the pecking order. Although my friend had told me he was the low-man in every group he had ever been in. So after a week I stopped seeing bites. But now it seems that I find a new bite on the 4 year old every couple of days. The weird part is if I take the 4 year old out of the pasture for a bath or grooming. The 7 year old will freak out. Then when I put the 4 year old back he will just start herding him around the pasture. And I do often see him moving the 4 yr old around the pasture. Should I separate them? How long should I wait to see if they will just figure it out? The 4 year old will not fight back he is very submissive. When they are separated the 7 year old looses his mind. Should I get a third horse? I tried just waiting and I do feed them separately. They always have access to plenty of fresh water and a round bale also grass 24/7.
Two is a really difficult number to keep at home because they tend to get very attached and insecure. With some horses, insecurity can manifest as aggression. Also horses definitely can shift from easygoing to dominant when their herd situation changes. The 7-yo may not know how to be a good, secure herd leader. Or the lack of a strong leader or a group of other horses to look out for danger is making him anxious. After all, they are herd animals and in the wild they rely on the vigilance of their herdmates to stay alive.
Warwick Schiller has some really interesting videos from the last few years on helping horses with separation anxiety. Basically they’re about teaching the horses to stay mentally present and be comfortable with just themselves.
I think you will end up needing another equine though. If you don’t want to own a third horse you could look into boarding a retiree or being a long-term foster home for a rescue. A standard donkey could also be a good option. Goats have worked for some people too. It sucks to solve an animal problem by acquiring more animals to care/pay for (BTDT) but the current situation doesn’t sound safe or happy for anyone.
If the 7 year old has been the low man in the pecking order for a long time, and he finally has a chance to boss another horse around, he is going to take it. In similar situations, I’ve seen otherwise submissive horses turn really mean, just like what you’re describing. I think of it like a power trip kind of thing. They’ve been beaten down repeatedly, so they tend to be a bit overzealous when they finally get the chance to boss around other horses. They don’t know how to be a fair, supportive leader, and the lack of a leader may make it worse.
2 horses just isn’t enough. They are herd animals, even if they haven’t been pastured with more than 1 or 2 others, doesn’t mean there wasn’t other horses on the property. Horses are very social animals, so when it’s just 2, they become extremely dependent on each other. The older horse is freaking out because you’re taking his support system away.
Personally, I would try to bring in another equid (donkey, mule, horse etc), at least one. Just make sure to plan ahead for when you travel that you’ve got a plan so they all go, or you have at least 2 at home.
3 is not always the Majik Number.
My TB used to herd his group of 5 geldings in pasture. They largely ignored him, quietly going wherever he decided they should be.
Then, for several years, he was Boss of a Herd of 2 with DH’s horse.
Then I bought a 3yo OTTB in partnership with a young woman who had share boarded my TB.
We put the 3yo in adjoining pasture & after initial squeal/strike by TB, they grazed peacefully separated by a fence, but right next to each other.
After several days of this, we decided to add the 3yo to the 2.
The minute I led the 3yo in, my TB came running, neck snaked, teeth bared.
TG my shout turned him away so 3yo & I could get back out ASAP.
Apparently TB felt he needed to protect his “herd”.
OP:
As long as bites are superficial, I’d leave your 2 to sort things out.
But be ready, if the aggression escalates, to keep them separated.
ETA:
At one place we boarded our 2, they were put out with 2 others, who - said BO - had a dominant gelding who was off-site with his owner for a week.
My TB assumed Boss duties & the others went along…
Until absent gelding returned & was put back in the group.
For about a week, TB came in with various bite marks on his butt, shoulder, neck. Nothing that ever broke the skin, but obviously he’d been reassigned his place.
All went well afterwards & no more bitemarks.
I have 3 and when I take 1 out the other 2 lose their minds.
I have found it just depends on the individual horse when having herd bound issues. I had just 2 for years and could leave 1 home and go riding. It certainly was easier.
Now I just confine the 2 left behind to a small securely fenced pen and let them do their thing and the horse I am taking away is fine.
I have 3 and it stinks as when I ride with one of my kids or a guest, a horse/pony loses his mind when left alone. During the summer my kids ride with me at least 3-4 times a year. So yeah that’s fun.
Really 4 is the perfect number cause you can rotate pasture buddies and still have enough that no one is alone.
Yeah, the perfect number really depends on the horses. Two of mine were great alone for over a year because the retiree doesn’t mind being left alone and never goes anywhere. Now I have three and that works also, but I know the two younger horses wouldn’t be okay without the third (unless I never took anyone anywhere).
Sounds like two is not working for the OP though!
I have two as well and it’s the retiree that loses his marbles when I ride or move the younger horse out of the barn / paddocks. The younger horse is pretty much unconcerned if I move the retiree. It’s an interesting dynamic. The younger horse has had lots of the Schiller work so there is that.
Isn’t it great when the unrideable horse is your hothouse flower??
I’m pretty sure it’s impossible for it to be any other way.
Useful days end and POOF, suddenly the horse becomes 30 times more labor intensive than any other creature on the farm. 
There is a stereotype about sensitive, chestnut TB geldings. I have the poster child. So high maintenance. Useless, but the sweetest boy.
Lots of rides with him hanging out in the “arena”.
My two must be the exception. When I take the mare out, my gelding does the horsey equivalent of “bye honey. See you when you get home.” He then goes about normal life until she gets back. He only gets upset if the trailer comes back without her.
