It could probably kill a dog too if endurance in heat was a factor since we have the benefit of heat regulation through perspiration that dogs don't have.

We are talking about 500 bpm. That's like 8 per second. In a huge heart.

I can't see how a big heart like that could pump enough blood to not faint immediately even for a human, let alone an animal several times bigger, and then, if it doesn't get pretty fast back to normal result in death.

I know the fibrillation part is an exageration, but sill, 500bpm?

Do you thing a human, let alone a bigger animal wouldn't instantly faint?

Edit: You can't compare a dog that is maybe 35-50 kg to a grown man who is 80 and then use this to talk about giraffes as you can't compare blood presure extremes to heart rate extremes.

A big heart has to have the time to fill up with blood to pump it. So, you have to have bigger arteries and higher blood pressure that can help with that, but raising the heart beat is only effective to a point were it starts to be to fast for the heart to expand. It is the reason why in humans aerobic exercise is advised to be up to 140 bpm (of course there are no absolute limits) since if you go higher the faster heart beat doesn't help circulate more blood.

So, a huge animal with a huge heart is very unlikely to have such a crazy heartbeat.That would be considered ventricular fibrillation and said huge animal would probably drop dead in a second.