When i bought my 4th horse 2 weeks ago my daughter (age9) tried the horse at the place we got her from and she was in a backyard in a town,they had a saddle older then my granny with a light bed blanket under her with a belt for reigns and a O ring snaffle too big for her mouth.and the people only spoke german to her...
and she was very quiet so figured if she can be quiet in THOSE circumstances she'd be an awsome horse for my daughter who LOVES to ride..i got her home and i put a GOOD saddle on her and a D ring snaffle her size and a THICK good non slip blanket and she did NOT respond to the bit and seemed to do as she pleased,not even to a whoa.so trying to do a lil ground work with her and she knows NOTHING! not even to lunge. odd time i can get her to go in a couple of circles finally then she stops and just wont move and if i have a whip ill tap her lightly and she doesnt flinch.tried some parelli on her and for the 1st 5 mins shes not to bad but then gets mad and will rear at me or try to nip..this is VERY unlike her personality cuz she will fall asleep on my shoulder usually..so its like she has enuff of training and doesnt want anymore but dont want her to think that by doing this its a way to get me to stop and she will always act out like this when she doesnt wanna do something..ill snap the whip and she doesnt flinch.just stands there and faces me i can finally get her to turn her front shoulders away but after a circle she stops again and again and getting a lil frusterated..not with her cuz she doesnt know any better..just like a few pointers on how to make her move her feet..been 2 weeks now so just thought id see a lil more progress then ive gotten from her.
sorry its long but wanted to fill ya in a lil on her history would REALLY love feedback on this and any help is appreciated..
thanks in advance!

Sounds like a respect issue. She will only do what she wants and if she's pushed outside her comfort level or to do more work than she wants she acts out. My mare would also rear and I just ignored it. Once you gain her respect that will disappear.

You need to get control of her feet. Get her moving, if you have a round pen thats a good place to start. If you can only lunge and her feet lock up on you (she decides she's had enough) get that whip and really crack her on her neck, drive her away from you. You shouldn't have to chase her rear around. If she still doesn't move whack her again but harder. You have to do what it take to get her moving. If you whack and she isn't going, you aren't whacking hard enough to make her feel uncomfortable.

Never whip the horse. read my thread of Respect Issues, it shoudl help its too long to re write!

I would recommend using a lunge line in a round pen. Do have a whip handy in case she does something dangerous, but don't use it on her unless you're in danger. It's definitely a respect issue! You said you bought her two weeks ago, so I am sure shes still adjusting to her new home. It'd probably be a good thing for your daughter to spend a good bit of time w/her, grooming and talking, basically forming a good tight bond w/her.
Have you ever done any training? It kinda sounds like you're gonna have to go back to the beginning and re train her to proper tack and cues.

Was she the only horse these people had? If so that could be part of the problem, she's not socialized. How old is she?

Good post JenJen.

Yep, it does sound like some behavioral issues, but then again she may need time to adjust to her new home. Or she may not have been trained correctly.

There is a difference between just "whipping a horse" and using a whip, or carrot stick in the manner of a true horseman. Remember, the WHIP is not BAD- it's all in how a horseman/trainer uses the whip. It can be used as a training tool, or it can be misused and totally ruin your relationship with your horse. I'd definitely go back to the beginning and build a strong relationship with her starting from some simple groundwork.

I aggree, my trainer has a horse that can be very mean and stubborn on bad days. I'm ganna be riding her soon, instead of the old horse I'm working with now. But we do have to use a whip on her because she has respect issues. If your walking by her pen, she could lean over to bite you. She has definitely come a long way since my trainer bought her, but the whip is a must so far with that horse mostly for our safty.

You did not say in your first post if anyone actually rode her in the backyard.
Sounds like a carnival/pony ride pony. They are very loving and will spend all day doing nothing, but haven't actually been trained to ride. Yes you can tack/mount and they move off without any bad/dangerous behaviors. But have spent most of their life being lead or tied to a walker with kids on them.
Best case if you and your trainer can, is just start over from ground 0. As if she was unbroke, the parts that were done correctly should go fairly easily. The spots that are lacking will take a little extra time.
Good Luck