Help! Unpredictable!! Dog Aggression!!

by Help!
(VA)

Help! I just got a dog from an animal shelter. She was listed at the shelter as a Schnauzer/hound mix. She is about a year or so old. She is perfect in every way, except... SHE IS AGGRESSIVE AND UNPREDICTABLE. I brought her with me to the barn (like, horses) today. We walked around and she was totally fine. I didn't get her close to any horses. I took her down to the indoor ring to play fetch (she loves fetch!). A girl, her horse, and her dog came down. I put her back on the leash and went to leave but the girl insisted she meet her horse and dog. she met both of them and was totally calm and didnt growl or anything to the horse or that dog. Ok, skip later. the owner brought his 5 m/o lab puppy over. the puppy jumped out of the back of the truck and ran and jumped on her. my dog had never met the puppy before and naturally i am sure she was scared. she growled and kinda snapped at her. i had her on a leash. she was kind of growling for about 30 seconds straight on the leash while she was meeting the puppy (who was actually pretty scared of her after she growled and snapped at her for running her over- i dont hitnk the puppy has ever had anyone be mean to her before ha!). they warmed up to each other and started playing. i had my dog on a leash still. then the puppy was grabbing the leash, and they seemed like they were getting along great so i unhooked it. they ran, wrestled, and played for a solid 30 minutes. they shared food (a piece of horse poop- yum,...), drank water from the same bowl at the same time, and laid together, too. then my roommate, who my dog had met in our apartment earlier that day (she had about 3 hours of being around my roommate, not a lot of time, but really likes her), came up to the barn. The puppy ran up to her and she petted it, my dog ran up to her, she petted it, and they continued playing. then, the puppy ran up to my roommate, and she started petting her. literally out of nowhere, my dog ran up and attacked the puppy. she was snarlig and biting her and being super vicious. i tried to stop her and ended up separating them, my dog was still going crazy and was actually (i hope unknowingly) biting my arm (not hard at all- didn't hurt). after taking some time in their own separate
corners to "chill out" we tried to reintroduce them (leashed), because well, they had been playing for so long before and getting along great. the puppy just wanted to play with her and wasnt exhibiting any aggressive behaviors even after just being attacked. my dog snapped right away and bit her lip, HARD. the poor puppy finally got pissed off and wanted to fight back. obviously we didnt let full on chaos insue!

i don't know what to do. i am a college student and have a dog at home (where i go in a month). I had seen met her at the shelter, saw her interact with a child (perfect- sat when he walked up to her with his hand out and just let him pet her without moving- i was impressed), and asked all the right questions. they knew i had a dog tht she would have to meet in a month, they knew i had friends with dogs who came over, they knew i rode horses and would bring the dog to the barn, where it would have to interact well with other dogs, people (and horses- though this is something that can be taught fairly easily). I have owned multiple shelter dogs in the past and believe me- I know that it takes some time to get them acclimated and socialized. having said that, i have never had a dog act so aggressively without warning and for what seems like no reason (she didnt care when i was petting the puppy- and she had only met my roommate a few hours earlier, so she shouldn't have been so possessive. ). have any of you had experience with unpredictable dogs like this? terriers? shelter pets? any advice?? i do not need her to LOVE other dogs, but i DO need her to tolerate them and frankly, an unprovoked attack is simply not acceptable. do you all think this is something that is "fixable"? how do you prevent a dog attacking another when it is not consistently dog-aggressive?? what would you do in this situation? this all just happened this afternoon. i am looking into reputable trainers in the area to get an evaluation and training for her (have calls into two recommended ones as of this evening- prob wont hear back until tomorrow as it is a sunday).

Thank you in advance!!