Almost every single basic mook in Dark Souls would qualify as a Demonic Spider in any other game (and you are very likely to die a few times during the tutorial) than this one, but there are some that stand out to the point of making you want to punch the screen in frustration. Without further ado, here are some of those Demonic Spiders:
The Basilisks who were immediately notorious after the release of the game. They have a breath attack which inflicts Curse, the most debilitating status effect in the game. The way status effects work, you have a meter that fills up as you are exposed to something that has a status effect. Players in their first play through will likely have zero resistance against curse, meaning the meter will fill up in about a second, almost instantly if you get caught in the cloud created by two of them. Curse kills the player instantly. When players revive, their health is permanently halved, even if they die again. Getting cursed removed is a quest in and of itself.
The Undead Balder Knights, who are very swift with their sword strikes, reliable shield, and above all, the ability to parry you. For beginning players, they're almost akin to a Miniboss, due to the low damage players will be dealing initially. The sole fact that they can parry attacks is frightening, as your mistakes are often punishable by instant death in this scenario.
There's a small copse of trees in one corner of the area that you fight them in that they can't move into. Standing in it with a spear, occasionally popping out to lure them back over makes the fight with them more tedious than challenging.
The large Mushroom People, to the point of exaggeration. After fighting the small ones, who are a joke (they even fall flat on their faces when they try to attack you!), their funny appearence and their laughably slow speed, most people will probably underestimate them and charge right in. The same people will then find out that these things not only have a metric ton of HP, but that their punches hit hard enough to easily kill most characters in one or two hits.
Skeleton Wheels. They're one of the few enemies in the game that are always faster than the player, and considering their main method of attack is rolling at you, this is a big problem. Blocking doesn't work well either; without either godlike stamina or a ridiculously good shield, you'll find yourself out of stamina alarmingly quickly, and then out of health. They're also surprisingly good at correcting their course, so dodging them is difficult without light equipment and very good timing. Weaving while running is about the best way to dodge them...until you enter the basement of the Painted Realm of Ariamis, and find them in tight, narrow corridors...
The Crow Demons of the Painted Realm. Tend to show up in groups of two or three, hit surprisingly hard, and they can jump on you and peck you to death. Worse, if you're trying to obtain Souvenirs of Reprisal without going online, they are the only enemies that drop them, meaning you will have to fight these things repeatedly.
To finish off the trio of awful things in the Painted Realm, we have the Bloated Undead, hollows with a massive tumor replacing their head. Aside from the same attacks as your typical hollows, they can shoot fireballs at you (!), and when they die, the tumor explodes into a huge toxic cloud. Do you have magic, ranged weapons, or a melee weapon with a good reach? You're probably fine. Do you not? Enjoy toxic damage.
The Serpent People residing in Sen's Fortress. Ridiculous strength, ridiculous damage reduction, and Sen's Fortress has at least one (more commonly a horde) literally around every corner.
Serpent Mages are worse. They throw lightning bolts at you, at a point in the game where your lightning resistance is probably nonexistent. Even worse, the game likes to put them in rooms where you have to cross narrow passageways and dodge swinging pendulums.
Channelers. They like to hide on platforms out of your reach and bombard you with Soul Arrows, which take off major chunks of health and tend to be very hard to dodge. Even worse, when you finally close in on them, they do their buffing dance and boost the attack power of anyone near them, including themselves (and they know how to use that trident). Even worse, they tend to teleport before you can finish them off, retreating about thirty feet away to rain Soul Arrows upon you once more.
In the same vicinity as the Channelers, the Crystal Hollows in Duke's Archives. They're fast, hit quite hard, usually attack in packs, and have a tendency to ambush you at the worst possible moment. It's likely you will die more often to them than the Serpent-Men of Sen's Fortress.
Torch Hollows. Take the basic enemy of the game, confiscate the broken and rusty sword it normally carries and give it a torch. Torch Hollows have earned as much respect and fear as the Wheel Skeletons to some players, and can do significant damage if it lands a full frenzy attack combo. The Depths bonfire typically hosts the numerous bloodstains of those that tend to forget how much of a threat the single Torch Hollow guarding the corridor out can be.
The Drakes in the Valley of Drakes. Massive health pool, powerful lightningbreath, and an all-around unpredictable A.I. pattern where they tend to take flight as you're about to strike them. They're even more aggravating due to the fact that they're the only respawning source of Dragon Scales (Hydras and Zombie Dragons don't reappear until the next playthrough after killing them in the current one), and even then they drop it at an absurdly low rate.
TheBlackKnights. As befitting of their intimidating stature, they all wield weapons that can break your guard easily and deal huge damage, and they have one of the highest aggro-ranges of any enemy in the game, meaning you'll have to defeat them or they might follow you around even as you retreat to a safe bonfire.
The Silver Knights of Anor Londo, while not as powerful as their black brethren, are no slouches, either. High defenses, impenetrable shields (much more pronounced with spear-wielding knights, as they will spend most of their time poking at you from behind their shield), and the ability to wield the massive Dragonslayer Bows, which can catapult you off your feet and are the main reason why one section of Anor Londo is widely hated by players.
Titanite Demons. While they often qualify as minibosses since none of them (bar one in Lost Izalith) respawn, not so much in Sen's Fortress, where getting hit by a pendulum throws you into a pit filled with them, giving you a taste of horror as you deal puny damage to their rock-solid skin and titanic health while they're about to crush you in one hit.
Blighttown's boulder-wielding Infested Barbarians. The regular ones are bad enough, wielding Large Clubs with added poison effect that can push you off ledges (which means instant death), but boulder-wielding ones have access to surprisingly long-ranged attacks (by throwing said boulders), rolling their boulders, and just plain swinging them. Not only they do a lot of damage, they have a very large aggro radius and can easily knock you down even if you sport a high Poise stat. They're such a problem that even experienced players opt to use a longer detour to avoid aggroing them at all.
Maneater Shells, in all honesty, are quite slow, very unbalanced, which means any hit from nearly any weapon staggers them (if not outright flipping them upside down), and do drop valuable items (Twinkling Titanite for unique weapon upgrades, and Purging Stones to remedy Curse). What people often forget is that their attack power is, for all intents and purposes, through the roof. Their body—er, shell slam attack can also throw YOU off balance, and they tend to spam them in rapid succession. Like other examples, their most popular farming place, just before Seath's second boss fight, have six of them spawn in very close quarters. Getting swarmed by them is basically instant death for unwary players.
The Stone Knights in Darkroot Garden. While slow and cumbersome, most of your attacks at this point will bounce off their armor, and they will cast the Tranquil Walk Of Peace miracle, which slows the player's movement to a crawl. While this annoying spell is in effect, it's very common for the nearby Ent-like Demonic Foliage to swarm you and start hugging you to death.
On this particular example, if you happen to be infected with a Gravelord curse while in New Game+, stronger Red Phantom versions spawn en masse in the same area alongside their original counterparts, making crowd control nearly impossible and thus making one of the hardest areas to fight your way through. Case in point.
Giant Skeleton Beasts in Tomb of the Giants. They lurk in the dark, are particularly ferocious, bite off enormous amounts of your health, and players will be in for a nasty surprise if they count on these monsters to stay stunned after an initial attack. Oh, and they're usually paired with a Giant Skeleton archer/sword-wielder (which are already quite deadly by their own rights) or stay in group. Their presence makes the tomb a nightmare for low-level players.
After a heated fight against the Sanctuary Guardian and a nice little rest near Elizabeth, you'd think there wouldn't be too much to worry while traveling through the next area, which is a near-exact replica of the future hunting grounds for the Forest Hunter Covenant. Nope. Be prepared to be swarmed by aggressive tree-like scarecrows wielding pitchforks and even shears, although they come off as little more than Goddamn Bats by the time you access the DLC. What's truly dangerous is the Guardians that are nearby. See the Stone Knight example above? Replace their weapon with a massive axe that can throw you off your feet, and you have yet another unnerving Mighty Glacier to watch out for, made even worse as they often come around by the time you're still busy fighting the scarecrows.
The Bloated Sorcerers in Oolacile Township. While they don't teleport away like the Channelers, they still pose a massive threat to you given their wide array of Abyss sorceries, near-instant poisoning if they reach out for you with their hand, and are usually backed up by several ferocious Bloatheads who don't show remorse in overwhelming you with sheer numbers.
The Humanity Sprites in the Chasm of the Abyss. Sure, they're relatively harmless if you don't approach them, and they don't actually have a lot of health. What most players forget is that they are very resistant to elemental weapons, and cause Collision Damage at an alarmingly high rate; if you're not careful when attempting to heal after the initial surprise, you'll end up swarmed by several of them.
Every single red phantom enemy spawned by a Gravelord infection becomes this. Increased health, attack power and, most importantly, aggro-range, causing them to chase you even more aggressively than you would initially assume, and sometimes before you even notice them. Woe betide a player who attempts New Game+7 and gets gravelorded along the way, as those same red phantoms will usually One-Hit Kill even players with maximum health.
May Gwyn bless your remains if you somehow desire to download the mods 'Perma-Gravelord' and 'Hyper-Aggressive Enemies' for added difficulty.