These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The first one was so awesome, it made it into I Wanna Be the Guy as the theme for the Final Tower of the Guy, continuing on for The Guy's first form!
Breather Boss: King Bee is considered MUCH easier than the previous two bosses and the following boss.
Demonic Spiders: Many enemies can be this, but the Tyrannosaurus and Allosaurus are the worst of the bunch, being able to cause massive damage very quickly, and also being nearly unbeatable if you're not tall enough.
Game Breaker: Boss fights become much easier once the player discovers that undergoing an evolutionary change also has the beneficial side-effect of also healing all your health. Just stock up on EVO points prior to the battle, and when you're getting your ass kicked, just go to your evolution menu and make one small change (like adding a horn), and you'll be back in business.
Goddamned Bats: Trilobites in the Fish Age chapter, most flying enemies if you're not a bird, and the monkeys and dino-people in the final chapter.
Hell Is That Noise: When you beat Bolbox, a very, very odd noise is repeated over and over. Considering how hard the fight was, this is a rare occasion where the noise is also the Most Wonderful Sound.
It Was His Sled: The Final Boss is a monstrous cell with an awesome battle theme. Thanks to I Wanna Be The Guy, this song is the first thing related to the game some newer players learn.
Nightmare Fuel: Bolbox can be this sometimes. The level with the cockroaches borders on it as well. As for the Queen Bee's face... ouch.
Older Than They Think: The SNES game itself is HEAVILY based on a much older game by Enix called 46 Okunen Monogatari: The Shinka Ron ("4.6 Billion Year Story: The Theory of Evolution"), released only in Japan in 1990 on a very obscure home computer called the PC-9801. E.V.O might even be considered a heavily-downgraded port, which is odd because as we all know, the SNES can handlesome pretty epicRPGs.
Signature Scene: The final battle with Bolbox. Mainly because of how bizarre the entire scenario is, the awesome music, and the brutal difficulty. In fact, thanks to I Wanna Be the Guy, this might be the first part of the game one learns about.
Which of the three yetis? The father yeti is somewhat problematic, but the mother and the son's descendant are the most problematic ones, especially the son's descendant, when you have to fight with him in the last segment of the game.
Oh, the yeti mother. If you're focusing entirely on agility, she can kill you in two hits, and a single hit of hers has a pretty big chance of being followed up by a second one, without giving the player a chance to react. Better yet, her pattern is chaotic. Sometimes she'll throw ice at you after you attack her, and sometimes she won't. Sometimes she'll jump before slapping you, giving you chance to react; sometimes she'll simply walk up to you at ridiculous speed before doing so, giving you none. Oh, and her recovery time is almost inexistent; it's guaranteed to be faster than any follow-up attacks or attempts to retreat, meaning she'll counterattack you pretty much every time unless you hit her at just the right stop. Most players will have to hit her at least ten times to kill; most of the time you'll be lucky to do it once.
The giant outside Bolbox's cave, the shark at the beginning and the Cockroach of Bolbox. The bosses in this game are abnormally difficult.
You may have a horrific time with the queen bee, even using cheat codes to stay alive.
Bird Man King is not afraid to utterly destroy you, being a threat even as the dragon. If you die, you need to do That One Level all over.
Mt. Brave is partly this trope and Brutal Bonus Level (it's optional). Birds pick you up and force you to start over. And the reward you get... is becoming a new class of evolutions, forcing you to start over. Unless you figure out how the Cloud Maze and River of Asteroids work, which give you the REAL reward: The Dragon and Gargoyle forms, along with a ton of Evo Points.