If you hang around with horse people long enough, you're likely to hear "He's just bridle lame."
What does that mean?
A "bridle lame" horse is one that appears to be lame or in pain, but is actually just off balance, crooked, lazy, or evading the rider's cues in a way that throws him off balance or crooked. It's often caused by a poorly fitting or harsh bit - or a bit the horse simply finds uncomfortable. The other common cause is laziness, a horse that refuses to step out properly under saddle.
Bridle lameness is a training issue - fixing it depends on the cause. It may require a change of bit, it may require work in ground lines to encourage straightness. I know one horse who is bridle lame unless you kick his butt to make him work (And has been known to pretend to be lame too - yes, some horses are smart enough to think of that).
Bridle lameness can sometimes fool even an experienced rider. If pushing the horse forward with the reins released evens out the stride, though, it's almost always bridle lameness.