It doesn't even just happen to humans. It's a known hazard in racing...sometimes a horse will just go down mid race from sudden cardiac arrest. Same thing, it's an undiagnosed heart condition. There was a high profile case a few years back where a top show jumper dropped dead in the middle of a round. It's not as common in horses, but it's a thing that riders are aware of.

It's also reported in dogs, although I don't know as much about it. I'd imagine all mammals are vulnerable.