What I did with my Border Collie/Lab (who had already been introduced to horses, but only enough that he thought they were rather scary and super fun to bark at) was I just put him on his leash and took him with me over to the barn. I used his leash to tie him to a fence post near wherever I'd be working and just left him there. That way he could watch what I was doing and realize that horses didn't need to be barked at and they were not as scary as he thought.
The most important factor for him, I think, is that he knows that I "command" a variety of animals and that he has absolutely no right to do anything to them that I do not allow. I'm not sure how he came to understand that but with my animals that set his prey drive off (the guinea pigs I used to have, my cat, etc), he stays as far away from them as possible because he seems to understand that being unfriendly to them is absolutely not allowed and that he cannot handle that rule when in close quarters with them.
Another thing that has made my dog LOVE Lacey, my horse, is going on rides together. At first I kept him on leash (he walks really well on the leash and just acted like Lacey was walking him) and pretended like I was ponying him just because I wasn't sure how he'd respond to any commands I had to give him from horseback. But once he showed that he's fine coming to me even when I'm riding, I let him off leash. He absolutely loves running so when he discovered that Lacey could run faster than him, oh man, coolest thing evvvvveeeeerrrrrr!
I'd advise against letting Cinny run with the dog until you're sure that the dog won't nip at his heels though. For me that whole "this horse is mine" thing works and keeps my dog from nipping at Lacey (and I think he's still too scared of her to think of doing that) but for a young, new-to-you dog I would be much more careful. Since you say she's laid back, that might not ever be a problem but some dogs are naturally really good at keeping their prey drive in check until something really really tempting appears.
Good luck! Going on trail rides with a dog is a great way to tire them out and a tired dog is a good dog! And congratulations on the new dog!
ETA- I second everything coffeeaddict said, especially getting that recall and "stick to you like glue" thing perfected. :)