Murlocs are from Warcraft, they first appeared in Warcraft III and have since reappeared in WoW, where they have gained a sinister reputation not unlike Carp in DF.
You see, Murlocs tend to spawn in large clusters in WoW, which tend to be rather dense and cover a fairly wide area, they're also group creatures, which means if one spots you and attacks, any Murlocs nearby also 'aggro' onto you and attack, furthermore they have a very wide radius to call for help, and a very wide radius to aggro, they also move quite quickly. Finally, they make a distinctive gurgling noise when they attack.
All of this combines to create a critter that is very unforgiving of mistakes; if you screw up and aggro the wrong Murloc you'll get dogpiled in seconds, Murlocs have claimed more lives than most Demon Lords due to their nature, and Blizzard has picked up on this and made the new Murloc species in the Xpaks even more like this. Northrend Murlocs are nightmares with froggy legs.
TL:DR - Murlocs are Warcraft Land-Carp.
Oh, and did i mention that they also have a fairly low respawn timer, so it's not uncommon to have the little bastards start respawning while you're half-way through their village, trapping you in a gurgling amphibious ring of death. Murlocs in WoW are not nice, i think i've captured their deadliness rather well here.
Their attacks aren't actually all that powerful (they're slightly smaller than Dwarfs), what makes them so ludicrously dangerous is the fact that they come in large numbers, run all over the map looking for prey and seem to move in packs.