We got a new horse for my daughter to work with. He's not a very young horse, but just very green, as in doesn't know very much... but soft mouthed and willing, so far. It's a project for her. Yes she's experienced enough to handle. Anyway no issues popping up under saddle, he's very wiling to do what she's asking majority of the time. We've only had him here maybe 2 weeks now.  He's not in the greatest condition.. but well enough to ride. We are feeding him up... getting him conditioned.. had his teeth and everything done by my go to vet .. he said he's not in terrible condition just needs more weight and muscle. Been dewormed, etc etc ... he's well on his way to getting right.  I have to say.. never in my life have I encountered a horse that doesn't want to be turned out! He walks  back to his stall and wants to stay in it!  We discovered some barn sour tendencies... which is weird because at his old home he was strictly on pasture only... never stalled ... he stayed out on their pasture 24/7 ... I don't do that here .. we have about a 3 acres pasture fenced off with really nice coastal and well maintained... he wants nothing to do with being turned out ... we lead him out there and he will eat Grass if you stand there with him but as soon as you go to walk away he follows... and then just cruises back to the barn and will stand in front of his stall wanting in!  We let him out again this morning and he didn't go far from the barn ... he only ate some grass for a few minutes and then wanted back in his stall. My horse is in the barn too but he doesn't go over to his stall like being buddy sour.... he just stands in front of his stall and waits. Its so odd... when my horse goes out to the pasture this horse doesn't care he's gone he just stands happily in his stall. When she rides him he will test her in the beginning to come back towards the barn but she gets him thru it every single time he tries ... he learned quickly there's a lot more work if he decides to come back over to the barn while under saddle ... he doesn't give too much of a fight. I do not have a fence to keep them out of the barn area from the arena and pasture... its all open.. we never had issues of horses not wanting to be out like this. 

I'm at a loss on this one. So wondering if allowing him to stay in the barn is going to contribute to his barn sour-ness or just leave him be... I don't like to pick silly battles ... I do not turn my horses out together because my horse is a jerk and will kick, chase and just be obnoxious and if he gets them cornered in the barn I'm afraid he will hurt himself or the other horse.. I've always done separate turnouts here. 

Shes doing a great job with him undersaddle ... making the wrongs things hard and right things easy .. he's catching on quickly. I just don't want to add to any issues with him staying in the barn.. but he seems perfectly content in there lol! Soon as the other arena dries up we will be hauling there to ride instead of at home.. but for now she's working him at home in our set up. 

Well since you are not turning them out together and one horse is staying pen up, the new horse to me is wanting to stay with your horse, since they are herd animals I can see him wanting to stay with another horse and plus you feed him in his stall, so food and another horse is there in your barn, so he wants to be there too. He dont want to be alone in the pasture is my guess. I bet if you turned them out together he would enjoy the pasture more. I would close off the barn area you have so that way he cant get ran into a corner. When my horses are out together I close off the smaller pens so nobody cant get cornered up. 

I would say he wants to be with a buddy.    

Other possibility is he doesn't like bugs, so wants to get away from them more indoors. 

I turned them together in the pasture a few days ago and he still came back and waited by his door. It's very odd. There are other horses out there we share a long fence line with our neighbors and she has 2. They are easily visible.  My horse proceeded to kick at him up at the barn and kicked him twice. My horse has been solo for over a year out there anyway .. but I know he's done that before to others we had .. they stayed away from the barn always .. its just this horse.. fencing my barn off isn't an immediate option but we had planned to cut off the pasture at the arena at one point 

I also tried feeding him out there this morning too.. thinking maybe that would help... nope... he walked back to the barn and stood in front of his door, wanting in. He's very patient about it. My horse lunges at him and pins his ears at his stall door down a few ... he's just a big ol jerk. Always has been like that. The horse came from my family member and I asked him what the heck.. he said he never stalled him ... only if the weather was really extreme, but he was definitely turned out more than stalled at their place. We have different climates too. We are definitely less humid and way less giant horse flies here than what they have... so im not sure if it's the bugs or what. I'm just going to keep taking him out and hopefully eventually he will come to realize it's not so bad out there. 

They want what they want lol.

My horses love hanging in their stalls all the time!  They have a choice 24/7 and majority time they hang in stall.  and they always face out to see outside!  But they do go out on their own every so often to eat grass and back in they stay.  They are happy they just look so relaxed and chilled! 

My weenie dog used to always stay outside.  Somehow my hubby sneaked him in a few times and now he doesnt want to come outside only to potty (doggie door)  I can leave the front door open and the other weenie dogs will come outside and play awhile while I feed and mess around but not him! He will stand at the door and look outside and not come out!  He loves inside!  I dont know if he afraid he will get stuck outside even tho he had a dog house and nice bed. lol 

If its just a couple weeks it may take him a bit to settle in.  I dont know your setup but if he has a choice inside and outside maybe he full and just wants to be inside since he not had shelter before.  If he has a choice Id let him pick what he wants to do. 

Maybe his quirk! lol

Yep.. he just prefers to be inside right now, I think. He's riding better and the silly, " I wanna go back to the barn" nonsense is wearing off.. he finally figured out it just means that much more work. Yesterday she was on him for over an hour.. she's determined NOT to go down that road again. And yesterday was the most quiet he stood after riding being tied up to the patience post .. no digging,  no jigging around.. just stood there like he's suppose to. I'd say she's making excellent progress. He's just very content being inside.. we've had some pretty nasty storms come thru since he's been here and I think he may appreciate the shelter instead of just trees. Plus the insects are almost non existent in my barn with the spray and fans going. He may think he's found an oasis lol! I'm not going to sweat him about it.. so long as he's behaving for her under saddle

Thanks for the replies! 

I don't have a barn - just pastures with run-in sheds. When I send my guys off for training the response to being stalled is very varied. My DFP stallion Peon is maybe the most extreme with loving his stall. It is his safe spot and he loves his 'home.' When he was here with all the wide open space and freedom to do what he wanted whenever he wanted I think he sometime got himself wound up. He crashed through my fencing twice at a dead run, like the fence wasn't even there! He's ok for a short turnout, but he gets too wound up if the other horses get to running. His little sister Demon does not like stall life, tho she is doing fine with part time turn out now. At first she was a little escape artist in the fields. Now that she has decided to stay in the correct paddock at my trainers she is happier overall in her training life since she gets to go out more often. 

He finally decided it was ok to come out this afternoon.. I just tied his door back and put a small gate across my chicken coop ... (my horse has let himself inside my chickens coop before to check it out and try to eat their feed) ..  he walked out a few steps and then turned around to go back in. I had my horse in his run and threw some hay out for both... he must have gone in and out 10 times before finally deciding to go a little further to the grass to eat.. while he didn't go far, he was out! Progress!! I was happy to see him munching some grass! It would do wonders for him right now. I just left him alone but kept an eye on him from my kitchen.  Hopefully he will get more confident as time goes by.