About the only real danger of the ones you mentioned is a scorpion under a rock you just picked up. They do hide in crevasses under rocks during the day. Just be aware that there might be a scorpion under any rock, and pick it up accordingly. For example, if it's a fist-sized rock, don't pick it up by wrapping your hand around it. If in doubt, kick the rock out of the way first, or move it aside with a stick or something.
As for your other concerns, they are either unrealistic, very unlikely, or downright silly.
Snakes are too large to hide under small rocks you can pick up with your hand. There might be a rattlesnake behind a rock obscured by a bush where you don't notice it. That is something you need to be careful of, but not under a rock.
Bobcats and coyotes simply don't attack people (assuming normal, not rabid, etc). If there is a bobcat nearby, you won't see it. If you do see one further away, it will keep track of you and stay out of your way. Bobcats are too small to see humans as prey. Coyotes might let you get within 50 feet if you're lucky.
On very rare occasions, mountain lions do attack people. However, this is almost always where civilization has encroached on their range, and where they have gotten at least partially used to humans. Mountain lion attacks do happen in the hills around Los Angeles, for example. It's not going to happen in the Nevada back country where the cat has its natural prey and a fear of humans.
I consider myself very lucky to have seen a mountain lion in the wild twice, even though each time was just a few seconds. But, I've spent many days in the deserts and other wild places of Arizona, New Mexico, Colorado, and Utah.
It's funny that the you didn't even mention the most relevant two dangers at all, which are dehydration and hypothermia.
Not only bring water, but something to replenish electrolytes. I use Gatorade powder, but there are lots of strategies. This has been discussed a lot here, so I won't repeat it. Go do a search.
You might think hypothermia is a silly thing to worry about in the hot desert, but it's more likely than anything you mentioned. If you're at high enough elevation, all it can take is a thunderstorm (often accompanied by hail) and wind. If all you've got is a wet cotton shirt, you can be in trouble in moderate temperatures and wind.
Even worse, if something happens so that you end up spending a night out, hypothermia is a real concern. Nights can be surprisingly cold in the desert, especially at elevated altitude. Clear sky allows for lots of radiational cooling. Couple that with a thunderstorm, some wind, and you can be in serious trouble with just a cotton T shirt.
As for finding gold nuggets under rocks: Not gonna happen. It doesn't work that way. I'm not even going to bother explain this one.
You should also think about permission and impact on the environment. Rolling over a few rocks by the side of the trail is probably OK in most places. However, going off trail and re-arranging the landscape may not be. Prospecting for gold may require a permit. Actually removing gold may be illegal, although this is mute since you're not going to find any.