The enemies in Mario games are almost as iconic as the plumber himself. Koopas, Shyguys, Goombas, and, of course, Bowser himself are all important to the franchise's identity. Nintendo definitely isn't afraid to put out new enemies, though. Mario Wiki's list of enemies has over two thousand entries - and that's not counting bosses.
When you put out that many new baddies, the majority aren't going to be recurring characters. No one expected the "Oucher Glass" from Mario & Luigi: Superstar Saga to be an all-time classic. There are some enemies that seemed to vanish for no reason after their debut. Maybe they came from a significant title, or maybe they're just too weird to get lost in the shuffle. Could these enemies have lost potential? There's always the chance they could appear again in the future, but for now, they're relics of the past.
10 Ostro
Ostros are large flightless birds that debuted in Super Mario Bros. 2. They're basically cartoon ostriches. Ostros are typically mounts for Shyguys, who chase after the player in an attempt to trample them. If the Shyguy was knocked off, the player could hop on top of the Ostro and ride it across obstacles.
Although they're enemies, Ostros were the first creature that Mario could ride, predating Yoshi. They were also a source of some confusion back in the day. In the Super Mario Bros. 2 credits roll, Ostro's and Birdo's names were switched. The real Birdo proved to be an enduring character, but all Ostro got was a few appearances in the Super Mario Bros. Super Show! cartoon.
9 Spikey
A rare enemy from Super Mario Land 2: 6 Golden Coins. Spikeys are only found in one level. These silly-looking hedgehog creatures will roll up into balls before charging at Mario. Despite their name, jumping on them doesn't do damage to the player.
Could Spikey be a jab at Sonic The Hedgehog? Probably not. The original Sonic The Hedgehog came out in 1991, the same year that Spikeys made their debut. It's very unlikely that Nintendo added these enemies last minute to rag on Sonic. Spikey hasn't appeared since its first game.
8 Milde and Marching Milde
Mildes are small, Kirby-like enemies from Super Mario World 2: Yoshi's Island. When stepped on, they pop like balloons and knock back any nearby enemies. Aside from that, Mildes don't do much aside from scuttle around in lines.
Marching Milde is the biggest Milde of all. She is the boss of her own fortress level. She threatens to pound Yoshi to bits, but can't do much more than pace back and forth. Ground Pounding her splits her into two smaller Mildes, which can also divide into a total of eight regular Mildes. Not the most high-octane exciting boss fight, but these things sure are cute.
7 Chuckya
Large purple relatives of the Bob-omb found in Super Mario 64. They use the same throwing attack as King Bob-omb and share a model too. Mario has to defeat them in the same way he did the king - by sneaking behind and tossing them.
Chuckyas are very frustrating enemies. If one of them grabs Mario, it's likely he'll be sent plummeting off a ledge or even off the stage. Maybe it's for the best that they haven't appeared since Super Mario 64. Chuckya may have gotten a shout-out in Super Mario Sunshine with the Chuckster NPCs. Chucksters are ostensibly allies, but they've probably taken even more lives than Chuckya.
6 Hot Foot
Hot Foots (Hot Feet?) are clever creatures from Super Mario Bros. 3. They look like little fireballs with legs and hide in candles until Mario comes close. For some reason, they follow the same rules Boos do. A Hot Foot will only move when Mario's back is turned. They can only be defeated by Hammer Mario or Starman-induced invincibility.
These are pretty unique and fun enemies, but Hot Foots never appeared after Super Mario Bros.3. Probably because their gimmick revolves around hiding in a candle. When was the last time anyone saw a candle in a Mario game? Not recently enough to justify bringing back Hot Foot.
5 Eyerok
Nintendo loves bosses with giant floating hands. Look at Gohdan, Wham Bam Rock, and even Master Hand. Eyerok is exactly like that, and it was a boss in the classic Super Mario 64. Almost every enemy from Mario 64 reappeared at least once in later titles. But not Eyerok, which is especially strange considering it's a boss.
Eyerok made one appearance outside of Mario 64 and its remake. It's a Mission Mode boss in Mario Kart DS. Eyerok may still be with us in spirit, though. Knucklotec, a boss from Super Mario Odyssey, is quite similar in theming and fighting style.
4 Strollin' Stu
Strollin' Stus were basically the Goomba stand-in for Super Mario Sunshine. They're simple enemies that don't do much other than bump into Mario. There are a handful of variations, like the firey Smolderin' Stu and the hat-stealing Swipin' Stu.
If you know anything about Gamecube-era Mario games, you know that Nintendo was really pushing Super Mario Sunshine. There was a period of time where Petey Piranha was in every spin-off title, and even in Super Smash Bros. Brawl. Despite that, Strollin' Stus have never appeared outside the game they debuted in.
3 Phanto
Phantos are one of the most memorable Super Mario Bros. 2 enemies, although not for the best reasons. These haunted masks chase the player whenever they pick up a key. They can fly through walls and will even chase through multiple screens and Subspace. They're fast and hard to deal with, and the only way to get rid of a Phanto is to put the key in its lock.
There have been a few references to Phanto over the years - a rug in Mario Kart, a Sammer Guy's name in Super Paper Mario. The enemy itself has never reappeared outside of remakes. It's a bit strange since Phantos are such unique enemies. Perhaps they'll return someday.
2 Goombo
No, not Goomba. Goomb-o. These are apparently distinct from Goombas, and live in Sarasaland, the setting of Super Mario Land. The only difference between a Goomba and a Goombo is that the latter is smaller. They otherwise look and act exactly the same.
Goombos have been so forgotten by history that it took decades for them to get an official English name. Fans only knew them as Chibibo - the Japanese name - up until the Virtual Console release in 2011. It is assumed absolutely no one rejoiced at the reveal of this information.
1 ComBat
A very strange enemy that appears in a few stages of Yoshi's Story. Mechanically, they're pretty forgettable. ComBats float around and will chase Yoshi until they get dispatched with a thrown egg. No, it's the ComBat's identity that's the problem. They don't really look like bats, do they? In fact, some sources refer to them as crabs.
If you were hoping the Japanese name would shed a little light, you're out of luck. ComBat's Japanese name is Asekaki-kun. This roughly translates to "Mr. Sweat" or "Perspiring Kid". This is both disgusting and makes no sense. Maybe this is one enemy that should stay gone.