Angband/YMMV


 * Demonic Spiders: Loads and loads of them. Zephyr Hounds, Quylthulgs, Drolems (the reason you never go below 2,000 feet without poison resistance), Ancient Multi-Hued Dragons, etc.
 * Goddamned Bats: Oozes, icky things, mushroom patches, and worst of all, worm masses.
 * Zephyr Hounds. Almost any time you see a "Z" on the screen (the earliest ones--Clear, Dark, and Light--are the sole exceptions) you'll want to cry.
 * Goddamned Boss: Smeagol. Wormtongue to a lesser extent, as he is actually threatening.
 * Any boss that summons. Especially when those summons can (and do) summon.