I have one who must have had one as a foal. Had routine radiographs as a three year old, and it showed up. They don't really heal completely on the radiographs (only bone that does that in the horse), so bone scans are the best way to determine if they are a problem or not.
Anyways, my horse was totally sound, and not sensitive in the least to hoof testers. Very bizarre. I figure it must have happened as a foal? He had never taken an off step in his life. Anyways, he's four and a half now and getting ridden six days a week. He's perfectly sound FWIW, it was on his left medial wing.
I don 't know about foals, but my horse broke his coffin bone when he was about 13. Put a bar shoe on for nine months, kept riding, perfectly sound FEI horse for 20 years. He was never off after that, sound until 31. I would imagine a foal has a much better prognosis. I think where it is makes a difference, but I think it's not usually a terrible thing.
Where it is and whether or not there is displacement makes a big difference. My mare fractured her coffin bone 11 months ago. It was a non-displaced wing fracture. Original prognosis was 4-6 months off work but it took 7 or 8 months for her to be completely sound. She was sound trotting on a straight line for several months before she got sound in turns and on a circle.
I would think the biggest challenge for a foal would be to stabilize the foot enough to allow it to heal. My mare had an eggbar shoe with multiple clips that acted like a cast to stabilize the fracture. And she was on stall rest for about 2 months and then sound enough at the walk to go out on a normal schedule. At that point, it was just a matter of waiting for her to get completely sound.
I rode a 3yo that never had any problems, until after we broke him and started riding. He came up severely lame, initially we thought it was an abcess in his hoof.
Then when it didn't get better (well worse in case of abcess ) we figured something else was going on and we had X-rays taken.
The X-rays showed that he had broken both coffinbones at the primeval process (?). The top part.
He will never be sound again...
I've had two foals who fractured their coffin bones. One had multiple wing fractures (her mother stepped on her). We put glue-on metal plates on their feet to stabilize them and stall rest. Both were sound in a few months and are sound riding horses. You cannot tell which foot they fractured at this point.
Study done a year ago or so at VT showed that 80% of warmblood foals radiographed had evidence of coffin bone fractures and that they all healed just fine.
No treatment was done in any of these foals. I think if you Google coffin bone fractures you can find the paper on line
Depends on the type and severity of the break. Minor extensor processor fractures have a high success rate of horse returning to full performance - support shoes, and perhaps pads, and in some cases, joint injections can help. There have been several studies on coffin bone fractures - as noted, they are relatively common in Warmbloods, and are not always an issue.
A couple of years ago we had a foal with a foot injury requiring x-rays. The x-rays showed that in addition to the injury she had a fractured coffin bone. She had never been lame on it, and when she recovered from her injury (nail puncture), she was completely sound.
Just a few days ago I read a study done on soundess after a fractured coffin bone. I think it may have been in The Horse. The prognosis for foals is excellent, and more guarded for adults with a fractured coffin bone.