Okay, here in lies a tale from my RF Online days...
In any game, I'm relatively good at killing stuff and getting stronger and death-dealing and so on. But this was at a point where I still needed to up my strengths, and so I turn my sights to bigger monsters.
This guy is larger than pretty much any humanoid in the game (there are three races to choose from). In a real-life confrontation, such a creature could bite you in half. It's like some kind of freakish armored dinosaur. Also, it's never alone. There's always a few others within reasonable proximatey.
And when I happened upon a whole entire cave full of them...well. It was fun for precisely a minute where I thought I've actually sneak by. Afterwhich, I was leading an entire parade of every single one of those damned lizards in the place. Because mobs in RF Online, they don't chase you and give up after a while. They follow you FOREVER.
Eventually, I got back to the home base, where the perimeter guns just shoot 'em down for trespassing. Now, there are probably worse mobs in the game, and there's definitely this mine guardian thing which people fight over control of, but my experience is the two dozen giant fucking vicious lizards who will pursue you for freakin' ever.