Thats not really true. Horses, while they like company of another, can get along fine alone. Thats no excuse for bad behavior- period. Don't cut your horse any slack- aggression is not tolerated with out without company. Sounds liek the trainer is looking to sell a horse.

Everyone appreciates some company.  A little companionship can prevent several bad habits, such as cribbing and pacing.  It has been a long-standing practice for race horse owners to provide a goat for company.  My horse has a large Boer goat as a companion.  Farmbabe is right, though.  Agression CANNOT be tolerated!

Marla

Your mare is trying to be in charge of the herd.. which is YOU and HER.  Sometimes I have to remind the top gelding in my herd that **I** Own the hay, when he tries to be too bossy with the other horses.  I chase him off the hay and let the other 2 eat. In your case ... you might need to carry a crop with you and when she gets bossy - smack her in the chest to make her back up. (I usually just kick the offending horse in the chest. My kick, with round-toed rubber boots is NOTHING compared to what another horse would inflict.)   I Growl at the horse(s) too.  (just like kids!! ha ha ha)

So far as being alone. I kept my gelding alone for more than a year. I got him some goats and he was grumpy towards them. Then I got him a pasture buddy. Doing that made him buddy / barn sour. But it was the buddy that would whinny and neigh when I took my horse away. I should have just left him as an only horse.  Now I have 3. sigh. 1 person, 3 horses & 2 goats.   

waaay   back when I Boarded my Horse, he was turned out in a  big pasture with 20 other geldings. He always kept to himself, never really  had a Buddy. so when I brought Him Home I had no qualms about keeping him alone and He did fine for years . I have Three Horses Now(  you seem to acumulate more when you have your own barn ) , all my Horses have learned to accept being alone at one time or another .   I can leave for  a weekend and not have to worry  if  the One Horse left at Home is doing Ok  . 

I agree that Your Horse NOW thinks  of you as  part of the Herd. someone has to be the Boss and she sure is trying to be the one .    I have also  carried a crop and stood over "my" haypile and  shooed them all away at one time or another, just so they don,t forget .

Personally, I think mini horses make better companions than goats.  Mini horses are far more fun too....just make sure your big horse gets acquainted with the little one before turning them out together. Miniatures are so fun, and in my opinion cuter than goats.  You can drive a mini, but you can't drive a goat.  At least he'd be a companion that you could use in some way.  On 1/2 an acre, I'd stick with a small buddy for your horse.  And as everyone said, the agression is proably coincidental.  There's no excuse for it.

Amanda

First - a goat CAN be trained to drive. The Alpines and Nubians are very good at it.

Second - yes goats can chew on horse tails. Knock on wood, neither of mine show any interest - the goats and horses all co-mingle in the fields. I use woven mesh fence and installed hanging boards below my gates to prevent their escape. Perhaps because I make sure they have loose Goat Minerals, loose salt, and baking soda, they have no interest in chewing on tails or other things.  They do pretty well on the weeds but not all of them. (I have pygmy and an Alpine. both does. DO NOT get a buck goat, they pee on everything - marking their territory)

Mini horses are fun (stop here! buy one!!) They may require a dry lot to avoid becoming obese and ending up with IR and laminitis/founder. They also live much longer than standard size horses.  Their fencing needs are a little different from horse fencing, i.e., lower to the ground if you use wire, tape, fiberglass belting, boards, etc.

Consider the foot trimming and alternative vaccinations required too. I was fortunate in that I found a VERY good farm vet who is also quite good with equines (and dogs and cats!!)  He can address goat & horse issues during his visits. (and stick the cat with whatever he needs).

So... if your horse can come and go into a barn with electricity - you might consider a radio. : )

First and foremost, OWN feeding time.  For the first several days, run her off her feed bucket for the first 20 minutes, then "allow" her in to eat.  This is lead mare behavior and will secure your leadership position in her horse-etiquitte mind.

Secondly, we kept our paint mare by herself for many months...she adjusted quite well to the idea, but was very, VERY happy to see us.  We did spend a lot of time with her, either in the barn or out in the pasture or riding. 

We have 6 dogs, and they tended to be good company for her.  An added bonus is that she was not spooky in the least when coming upon loose dogs out on road rides.

My youth show gelding liked being alone.  He could have cared less if other horses even existed.  I think he liked being the "one" that got to live in the show barn and get pampered...I readily admit he was a total priss.  I did have a barn cat he liked, she would go and sleep under his hay manger and they both loved the radio I played 24/7.

I would be very careful about your horse becoming overly attached to another horse.  I've seen this turn out to be the worst solution as they become inseparable.

You can quickly cure the feeding time problems with a little adjustment.  I used to make adjustments with the graphite shaft of a golf club, it offered good grip and length yet was very light and easy to carry.