My horse goes insane when the vet comes...?

Vets are working at a definite handicap when it comes to horses, because everything they do is scary, painful, or unpleasant, so it's not really surprising that Roper wasn't that into it. What is more surprising is that she stood for most of the exam but went apes**t when the vet went to look at her teeth. What this tells me is she's uncomfortable with having people touch her lips and lower face. What you should do is focus on rubbing her lips and just touching her face a lot and putting your fingers actually in her mouth and keeping them there until she stops chewing and trying to lift her head out of the way (my horses let me grab and hold their tongues--that might be taking it a little far, but it has come in handy more than a few times) and moving her lips around. If you're just standing there, rub her on the face until she relaxes, and have a five minute session before and after you ride. She may be tense at first, but when she relaxes, give her a break from that and go on to something she is comfortable with. Do this consistently, and try to get your friends to help you out, so she gets used to strangers touching her face as well. I had to do this with my OTTB for a very long time before she'd stop trying to nip me or me with her head when I touched her lips and mouth. That's the only reason I could think of that Roper would flip out, but of course there could be other reasons.