I thought of another example. I read an interview with Larry Jones when Hard Spun was racing. He said that Hard Spun was a nice, pleasant horse and he rode to/from the track in the mornings like a trail horse. He said that Hard Spun could easily be a pet. When I saw Hard Spun at Darley, he still seemed like a pleasant horse. The stallions were out in their paddocks when we visited and Hard Spun was one of the few that came to the fence to say hello and be petted.
FWIW, I have a gelding by Devil His Due (Halo in his pedigree). My horse is really sweet and kind...until he is NOT. He can be quite aggressive, and he can switch from very sweet to downright evil, and then right back again...quick as can be. He despises small animals - dogs, cats, etc. and will go after them if not watched/corrected.
I've owned him for 4 1/2 years, love him, and have learned how he ticks. We make it work, but I've had to put a lot of effort into figuring him out. Most people don't believe me about his "evil" side until they see it. He is sometimes so quiet and kind that a child could ride him. Other times, not at all. I can w/t/c on him bareback (sometimes...other times, I can barely walk him to the ring or paddock). He's very interesting and has taught me a LOT about working with a difficult horse. He's also incredibly intelligent, so when he uses that intelligence for good, he is the best horse I've ever ridden.
He would not be a safe or fun choice for the average child or amateur, as you have to be on your toes 100% of the time. I've rearranged significant aspects of my life to manage him properly (he has unique riding schedule needs, particularly in winter).
I've done a lot of looking into his pedigree, and have found that many people make similar comments about horses from the Halo line. One person described it best in saying that they have "light switch" personalities - fine, fine, fine, but if you "cross" them, WATCH OUT! Half the time, I don't even know what I've done to suddenly anger him. But I always know it is a possibility that he will become suddenly angered (often by something as simple as a cat meowing and walking near him).
I visited Clairborne back in '98 or so and we were warned away from STG as well. Unbridled however was a doll. They brought him out and we were allowed to pet him, take photos, etc he was calm and sweet. Also got to see Danzig, Mr Prospector, and several others that I can't remember the names, but one was a KY Derby winner, and one was the Belmont winner that started Woody's streak. I had always loved Unbridled so I was very happy to get to see him at the farm.