Dark Souls is known for its difficulty, but the developers didn't make a game simply to make it hard. FromSoftware understands how to reward the player, but at the same time doesn't believe in holding the hand of the player at the expense of challenging them.
The franchise is known for its brutal bosses, but there are plenty of normal enemies and mini-bosses that can feel just as challenging if not more so than a typical boss. It really comes down to player skill and confidence and the downfall or death of the player is often due to a lack of patience or a sense of overwhelming arrogance on their part.
10 Mimic
The beauty of games developed by FromSoftware is that they never feel the need to hold the hand of the player. They'll often go out of their way to punish players for getting too complacent and/or comfortable. In Dark Souls they don't introduce the concept of the mimic chest until Sen's Fortress. These creatures are meant to look like normal chest and contain an item inside, but once opened will stand up and fight the player. They're rather difficult and if they manage to grab the player it often results in death.
9 Basilisk
At first, Basilisks look like nothing more than goofy frogs with large eyes. The moment players release the toxins these creatures let out can curse the player and kill them out of nowhere they become more and more terrifying. Most people often confuse their anatomy, as well as the large orange balls, are decoy eyes. Its real eyes sit closer to their mouth and are much smaller. They hang out in packs, so the real challenge for players is trying to take them out while managing to avoid getting cursed.
8 Wheel Skeleton
Players who decide to enter the Painted World of Ariamis quickly come to understand just how brutal the Wheel Skeletons can be. They're exactly like they sound, skeletons that are tied to wheels and roll at the player inflicting successive damage over and over.
A confident player can dodge an attack with a well-timed roll, but when they attack in unison it can result in a quick death. The worst part is that even when the player dies Wheel Skeleton's can be seen running over the corpse of the player as if they're still a threat.
7 Crystal Golem
Among the numerous experiments Seath the Scaleless tried and abandoned are the Crystal Golems. Players are most likely to first encounter these lumbering beasts at Darkroot Basin but will encounter them twice more in both the Crystal Cave as well as just outside of the Duke's Archives. A patient player will never have any issues with these things, but they're the type of enemy that players can become complacent with and fall into a mode of going through the motions. This can then turn into a few good hits by a Crystal Golem that is paired with a ranged attack from a hydra in the water.
6 Giant Skeleton
The Giant Skeleton is an enemy players get pretty familiar with in the Tomb of Giants. They're what they appear to be, larger more powerful versions of the normal skeleton enemies, but a bit slower and clumsier. Like most larger enemies in Dark Souls, they hit pretty hard, but players that are nimbler and agile can effectively take them out provided they're patient enough. Their weapons inflict bleed damage which can take the player by surprise if they don't pay attention to the meter filling up in the heat of the moment.
5 Darkwraith Knight
Not only do the Darkwraith Knights have some of the coolest designs in Dark Souls, but they also happen to fight in an unorthodox way that's rather off-putting. Their focus is always on draining the humanity of the player and does this with a grab attack that's telegraphed and can be avoided. Darkwraiths are fear-inducing from an aesthetic standpoint and prove that the way something looks can be just as intimidating if not more so than what it's actually capable of. Don't let that fool anyone though, as they can still murder the player in a second.
4 Silver Knight
Players may look at Silver Knights as less difficult Black Knights, but despite having less health and strength than their black-armored counterparts they're still annoyingly difficult. The real issue is when they have a bow in their hands and they're shooting large spears at the player.
Dark Souls does a good job at making the player pay for being overconfident. The point of the Silver Knights is to make the player feel as though they're facing weaker knights without understanding that a weak knight is still powerful and more than capable of killing the player.
3 Drake
Without having level numbers above enemy heads Dark Souls still manages to clearly notify players when they've entered an area they shouldn't be in. One such case is the Valley of the Drakes which serves as a secondary hub that allows quicker access between areas. The problem is that it is filled with giant blue drakes that attack the player. Each one feels like its own mini-boss and the area is filled with several of them. This area is great to come back to when the player is higher leveled and is great for releasing serotonin and feeling good about one's self by killing giant drakes.
2 Large Mushroom
The Large Mushroom people that can be found in The Great Hollow and the Artorias DLC move rather slow and seem like gentle giants, but that's a sneaky façade on their part. They hit like a truck and have a weird pace to their strikes that can catch players off guard. They also happen to have a good amount of health so taking one down is more about patience than anything. These mushrooms attack in packs and the large ones will often be surrounded by smaller weaker mushrooms that act as nothing more than a distraction.
1 Great Feline
When Dark Souls players come across Alvina of the Darkroot Wood and learn of the Cat Covenant they become comfortable with the cats of the world. It turns out this was a false sense of comfort as the other large felines aren't as welcoming as Alvina. The Giant Cats located in the Darkroot Garden are large powerful beasts that also happen to have a ton of speed. They don't allow the player to heal and tend to attack in an all-out berserk fashion that can quickly overwhelm Dark Souls newcomers. They're also a tough out for veterans of the game as well.