you would want to know if it was pain related.
A horse is not allowed to bite you, even if in pain. But, she may BE trying to say that something hurts, so please do check that out.
my friend's horse was and is girthy. he had worn an illfitting saddle for years prior to her buying him, and that expectation of pain made him defensive the instant that saddle was put on his back. And, it turned out he had ulcers, so the girth put pressure right where it already hurt.
once treated for the ulcers, he was a bit better, but still has an attitude. what they have done is set the limit RIGHT AWAY. that means, when you walk in, if he give you or the halter or the saddle a pissy look, you verbally scold him and you make him move his head away from you, and it that doesn't do it, you make him move his feet. To that end, she did not tie him for saddling for a long time, but kept the leadrope draped over her arm, and while girthing (yes, slowly in small increments), if he even showed the thought of bringing his head toward her, she'd use the line to move him around, keeping her hand up to block his head. Then, he got a bit less like this, and she did not have to move him, but she still keeps one hand ready to block him.
He is allowed to clack his teeth (an old habit) and lower his head, but never to bring it around. it must stay facing forward. The sooner that is corrected (while the head is just barely moving off of the position is should be in) the better.
Now, this is what she did. He is STILL pissy in attitude. But, he keeps his head to himself.
of course, I forgot to say that a well fitting saddle was promptly purchased, too.