CastleVania games are full of loads of different creatures to kill. These guys though, are the ones that make you go "oh, shit"
The Guardians (of which there are 4) can be found in the Black Marble Gallery in Symphony of the Night. The fact that they're palette swapped from similar enemies may give you a false sense of security until these beasts start kicking your ass. The Guardians can take and give out massive amounts of damage. To make matters worse there aren't any save points nearby unless you've already collected all the pieces of Dracula. Of course, they are also a great way to level up, if you have the skills to kill them.
The Chupacabra is called the Cave Troll in the American versions of CastleVania. This guy first showed up in Symphony of the Night and he was a simple, easy enemy found in the reverse caverns. But when he showed up in Dawn of Sorrow he was a bit tougher. And even tougher still in Portrait of Ruin. By the time these guys showed up in Order of Ecclesia, you just wanted to avoid them at all costs. Capable of killing even a very high level Shanoa in one fell swoop, you can't tank these mofos. Gotta have skills.
3. The Tin Man
In the forbidden library section of Symphony of the Night, there were three enemies named after characters from The Wizard of Oz. The Lion, The Scarecrow and The Tin Man. None of them posed an immediate threat. That is until this tin shithead re-appeared in Order of Ecclessia and made you shit yourself and run for another screen the second he showed up. What's amazing is he does the same exact attacks, he's just a lot meaner this time.
4. Beam Skeleton
Also called Nova Skeletons. By the time you run into these fuckers, regardless of which game it is, you're quite used to fighting skeletons and a Blue (or Green) version doesn't seem to pose an immediate threat. Until they unleash their beam on you however. The worst version was in Symphony of the Night, as the bastard did MORE damage to you as you leveled up. The bright side is in Order of Ecclesia, his glyph (Nitesco) was extremely useful and easy to grab.
5. Lesser Demon
I'm not talking about the Lesser Demons that show up in both Lament of Innocence and Curse of Darkness, nor am I talking about the one that shows up as a boss in the Library section of Symphony of the Night. But rather, the fact that in that game, they later showed up as common enemies. Now, the boss version was an easy fucker, and the only notable thing is he can summon lesser enemies, and it's the only place in the game you'll see a Mudman. When they show up as common enemies in the Necromancy Laboratory, they haven't gotten any stronger. Except now they try to summon all the time. And they summon other Lesser Demons. There's one infamous room where if you do not act fast, you can soon be swamped by as many as 20 of these fuckers at once. Did I mention that they are invincible while in the summoning animation?
Whether they're called Marionette or Killer Doll, these things pop up in most CastleVania games. The ones you gotta worry about though are in Circle of the Moon. They have a habit of sluggisghly floating around as usual and then suddenly lunging at you. As in most games, their touch curses, and Circle of the Moon being quite hard and the only uncurse potions you can find being dropped from enemies (there is no store in that game!) they can be a real hassle.
7. Final Guard
No matter which game they show up in they represent one thing: Awesomeness. And they will fuck you up if you're not prepared, with quick reflexes and a weapon with decent reach. They're pussycats in Aria of Sorrow but wait until you run afoul of one in Order of Ecclesia.
8. Iron Golem
Iron Golems have been around for a while and I'm not talking about the pathetic boss in Circle of the Moon. These guys also appeared in Dawn of Sorrow and Portrait of Ruin but the one I'm talking about is in Aria of Sorrow. This fuckface has 9999 hit points and all attacks and magic do 1 hit point of damage to him. killing him will either take a lot of time or some cleverness (there's a soul in the game that will swap HP and MP, and this guy has no MP). It's worth it though, if you get this fucker's soul, you can actually transform into one, and tear ass throughout CastleVania.
9. Crossbow Armor
Yet another suit of armor, this one with a Crossbow. He showed up in Akumajou Dracula/CastleVania Chronicles but the one I'm talking about is in Portrait of Ruin. That crossbow doesn't shoot arrows, it shoots a goddamn missle, and it does a lot of damage. Sometimes you can smack the missle out of the air and it'll land on him and kill him. In fact, after some practice, these guys are no big deal. But we all remember the first time we ran into one and he turned Jonathan and Charlotte into bloody smears.
10. Flame Demon
There's Flame Demons in nearly every CastleVania game, the "Leviathan" enemy in Dracula's Curse being the earliest example. The one in Cricle of the Moon is a son of a bitch as Nathan will have to swing his whip around maniacally to avoid it's big meteor shower attack. All the elemental demons in Circle of the Moon are bastards, but this one is the biggest bastard.