"I had half a mind to explore the ancient ruins outside Westmarch, and then I realized they were crawling with dreadful scarabs! Of course, these creatures do not frighten me, but they make my research much too time consuming. And I am widely known for my efficiency, am I not?"— Abd al-Hazir(src)
Scarabs are Beast monsters found only in the Nephalem ruins near the Westmarch, i.e. Corvus and others. They crawl out and attack the player(s) en masse. While it is difficult to look at them in detail, they actually appear more like scorpions than scarabs: they have pincers, and attack with spike-tipped tails.
Scarabs do not have any special abilities and are very weak, but attack in larger numbers than any other monsters in game. They literally pour through every hole, or even over walls, surrounding heroes in a matter of seconds, often attacking from unexpected direction. Area of effect skills will quickly dispatch them, but be sure to survive until the end of the wave.