Nowadays, a game is pretty much required to have a solid replay value to be well received. Having multiple difficulty levels is a simple way to prolong the life of the game, but then Numerical Hard is to be avoided at all costs. So a common way of increasing difficulty is to put the player against tough mooks earlier and more frequently in the game. And then there are games who not only do that, but create news enemies that you will meet only in difficulties above normal, thus surprising players who thought they had already fought everything. Of course, expect these enemies to be Demonic Spiders or Boss in Mook Clothing. Some games make it so that some tough enemies don't appear in diffculties below normal, but then it's more a case of Easy-Mode Mockery. Compare Level Scaling, which has a similar effect but minus the difficulty setting angle. Also compare Replacement Mooks, a sort of narrative equivalent, and Underground Monkey.
- In the original The Legend of Zelda game, Red Bubbles (which take away your sword permanently if they hit you) only appear in the Second Quest (that game's version of a Hard difficulty).
- In Lollipop Chainsaw, there are named zombies you need to slay to complete an album, and some of them only appear on harder difficulties: 
- Ninja Gaiden Black, and by extension Sigma, has the Ryu clones, mini-bosses who appear unexpectedly where you fought common enemies in Normal mode. Then in Very Hard, you meet a new type of Black Spider ninjas throwing Fuuma Shurikens at you. And then in Master Ninja mode you meet yet another type with swords that literally burn you when they hit you. Hard mode and above also put you against tougher versions of the cops and soldiers, specially immune to some of your techniques.
- Bayonetta: Gracious and Glorious are more powerful versions of Grace and Glory that appear on higher difficulties.
- Inverted in Diablo II. In Nightmare and Hell difficulties, most of the normal enemies in Act V are replaced with "Guest Monsters". These are more powerful versions of monsters seen during all 5 acts of the game, sometimes with different names. Guest Monsters often have some palette shifts, and a few have new skills or abilities as well.
- In the first episode of Redneck Rampage, Hulk Guards only appear on higher difficulty levels.
- A variant in Blood: the appearance of mini-boss (degraded boss) enemies depends on the difficulty level. On the easier modes, only the first boss, the Stone Gargoyle reappears as a mook regularly, and the second one, Shial the giant spider, does so once or twice per game. On the harder modes, Shial becomes a regular encounter, and also Cerberus, the third boss (who had no mook appearances on the easier modes) appears once or twice as a mook.
- Star Trek Online: The PVE raid "The Breach" has a stage where you have to blow up structural integrity field nodes on the exterior of a Voth fortress-ship. In the "elite" version of the raid, the field nodes are guarded by a Bulwark-class battleship in addition to the beam and missile turrets they have on normal.
- The Game Boy version of Daffy Duck: The Marvin Missions had a special series of hard levels that you can play though if you beat the first screen on a Speed Run. These levels feature a few enemies that don't appear in a normal playthrough.
- In Kirby's Dream Land, the Hard mode replaces the mooks and bosses with the stronger counterparta of themselves. This is repeated in Kirby Super Star Ultra with Revenge of the King being the harder version of Spring Breeze (and both are homages to Kirby's Dream Land).
- Super Mario Bros. 1 downplays this, as beetles show up in the regular games, but when you play the New Game+, which is replaying the normal games but harder, goombas are replaced with beetles.
- In Super Mario Bros Crossover, it's possible to change the settings to replace the Goombas with not only Buzzy Beetles (as in the original game's second quest) but also with Spinies and with a completely new-to-the-original-game enemy, Spike Tops (which act as both Spinies and Buzzy Beetles at once).
- Jazz Jackrabbit allows the level author to mark any enemy as requiring a specific difficulty.
- Most of the enemies in Phantasy Star Online Episode 1 get replaced in Ultimate Difficulty by different enemies with the same basic behavior, but are usually faster, stronger, and possibly have a new elemental affinity or attacks.
- A lot of named aces in Ace Combat Zero: The Belkan War only appear on Hard, Expert, and Ace difficulties.
Hello, Unknown Troper. You'll need to get known to lend a hand here.
Three days must pass before this YKTTW is Launchworthy or Discardable