I've only ever seen one horse who I would truly consider "crazy." He was an older grade gelding, mid-teens age wise, I'd guess, and a friend of mine used him for 4-H gaming for a year (my friend was "between" trained horses at the time). That horse was a gem on the ground, just an absolute sweetheart to be around. He could be tacked up with no fuss, nicely accepted the bit, girthing, etc.
That gelding could be relied upon to absolutely explode every single time someone put their foot in the stirrup. Like flipping a switch, Jekyll to Hyde. Saddle didn't matter, bridle/bit didn't matter, weather/season/location didn't matter, every rider every time an explosion of bucking, bolting, running sideways into whatever happened to be there. That horse would run and carry on until he fell down rather than calm down. My friend tried to just ride him out once, just let him run in an open field, and after half an hour of straight crazy had to spiral him down small enough to bail off and go back to the barn.
A typical gaming pattern ran thus: Lead horse to gate, clear innocent bystanders away, and quickly and carefully climb aboard. Use gate to funnel horse into ring, and hope for the best. On a good day, he could be pulled by one rein at a time around barrels, and circled to a relatively slow sideways hop at the end of the run. A bad day, well... lots of time penalties.
I've seen a lot of spoiled horses, a lot of minor problems, a few major problems, a lot of "mad gamers," and that one that I would classify as crazy.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown