Humans are Terrifying

More seriously, humans do have a number of advantages even among Terrestrial life. Our endurance, shock resistance, and ability to recover from injury is absurdly high compared to almost any other animal. We often use the phrase “healthy as a horse” to connote heartiness - but compared to a human, a horse is as fragile as spun glass. There’s mounting evidence that our primitive ancestors would hunt large prey simply by following it at a walking pace, without sleep or rest, until it died of exhaustion; it’s called pursuit predation. Basically, we’re the Terminator.

(The only other animal that can sort of keep up with us? Dogs. That’s why we use them for hunting. And even then, it’s only “sort of”.)

Now extrapolate that to a galaxy in which most sapient life did not evolve from hyper-specialised pursuit predators:

• Our strength and speed is nothing to write home about, but we don’t need to overpower or outrun you. We just need to outlast you - and by any other species’ standards, we just plain don’t get tired.
• Where a simple broken leg will cause most species to go into shock and die, we can recover from virtually any injury that’s not immediately fatal. Even traumatic dismemberment isn’t necessarily a career-ending injury for a human.
• We heal from injuries with extreme rapidity, recovering in weeks from wounds that would take others months or years to heal. The results aren’t pretty - humans have hyperactive scar tissue, among our other survival-oriented traits - but they’re highly functional.
• Speaking of scarring, look at our medical science. We developed surgery centuries before developing even the most rudimentary anesthetics or life support. In extremis, humans have been known to perform surgery on themselves - and survive. Thanks to our extreme heartiness, we regard as routine medical procedures what most other species would regard as inventive forms of murder. We even perform radical surgery on ourselves for purely cosmetic reasons.

From a forum, why humans make a great monster race for other aliens to fear. Yet to find original reference yet.