Freakazoid Transformation

In a clever parody of the classic Wolfman transformation, Dexter shifts between several gruesome faces while changing into Freakazoid in the episode "House of Freakazoid." The individual portraits appear to be painted, a striking contrast from the show's typical animation.

The 5th face is actually Emmitt Nervend. An ongoing easter egg of sorts, the end credits of Season 1 challenge viewers to find the strange little man by telling them how often Emmitt appeared in the episode.

Cafe Aliens

From the Transformers episode "Starscream's Ghost"

There are two different cafeteria scenes in the episode, but both are populated by the same aliens. First we see this odd duo:

First appearing as patrons just standing around not doing anything, later on we see they've become chefs!

This duo ( or another pair of similar aliens) appeared in an earlier episode as prisoners, though they're both white-skinned then:

In the front of the crowd. Also worth mentioning, on of the gray robots in the back appears to lose it's body as it escapes!

Another alien patron, this alien has no apparent mouth, just a tube in its place. Early on it only grumbles, but later on it mistakenly calls a would-be assassin "Waitress!"

I'd question how the tube-mouth alien associated laser guns with food delivery, but soon after this odd fellow eats the weapon. Even though it sparks and smokes in his gut, he enjoys the gun, if thinking it's heavy on the photons.

While purely crowd filler (no lines or even movement) the vaguely fish/lizard-looking alien appears twice, both times seated next to the tube-mouthed thing.

A different pose, wowser!

The Informant

From The Amazing Screw-on Head (quite possibly the greatest thing ever)
Hanging out on a gnarled old tree in the sewers of Marrakesh, the Informant spends his time sending out Morse-coded messages.

The Informant ticks off previous employer Emperor Zombie by telling President Lincoln the last known whereabouts of Zombie's hostage. Zombie sends his vampire minion Patience to rub out the Informant, and she has a rat burn the Informant to a crisp.

Patience also uses the rats for a chair!

Being incinerated isn't the end of the Informant, rather the mummy body was just the host for his true form.

As hinted at by his double-voice, the Informant is actually a pair of scarabs. Fez and monocle wearing scarabs, no less!

The Informant Informants are inadvertently saved from a crushing death by shoe by Screw-On Head himself, who doesn't seem to notice them, and presumably escape.

Cape-Bat

Gentleman Jim Craddock transforms Batman's cape into a monstrous bat in the Batman: Brave and the Bold episode "Trials of the Demon!"

The cape-bat appears to have a body, though it may simply be a flap of cloth giving the illusion of one.

Conveniently, Batman kept his cowl.

After a brief tussle, the cape-bat is destroyed by Batman's ally, Etrigan. The fight ruined Batman's suit, so Etrigan once again comes to his aid and magically gives him a new suit (which may seem familiar to some Bat-fans).

The whole event is apparently just an excuse to homage old comics. And there's nothing wrong with that!

Howler Leeches

From the Angry Beavers episode "Omega Beaver," Daggett believes the world is being invaded by Howler Leeches after reading a comic book (it's 100% 37% true!). Evidently, the invasion consists of three phases:
First, the Leeches cause an Ice Age; Second, they land on Earth in atomic, blood-powered spacecraft; Third, the Leeches devour all life, which take 35 years to fully digest.

The Howler Leeches are coming

Send more money, I'll send more stuff!

Daggett's brother Norbert and some of their friends decide to pull a prank and dresses up as Howler Leeches to scare Daggett. Their costumes are wonderfully terrible, consisting of just a black sleeping bag which doesn't cover their faces.

Daggett assumes his friends have been eaten and are slowly digesting. How delightfully clever!

The comic company offers Daggett help by way of special offers --utter junk that doesn't at all do anything-- such as "attack robot beavers" (mops with bits of twisted wire), stale chewing gum, and a rabid dog. While an absolute scam, the company accepts poorly drawn fake money.

As it turns out, Howler Leeches are real, and they have a real wicked design. Hard to see in the screen-grab, but the Leeches have a secondary jaw similar to Giger's Xenomorph, which is what they use to talk (I assume it's a second jaw, for all we know it's a recent meal's!). In an interesting twist, they attempt to keep the rabid dog as a pet.

I love how the fake Howler Leeches look nothing like the real ones. Both designs are fantastic in their own way.

The atomic, blood-propelled UFO the Howler Leeches use is also nifty:

I wonder where they get the blood to propel the UFO.

Spiders of Mirkwood

In particular, their depiction from the 1977 Rankin/Bass adaptation of Tolkien's The Hobbit.

For the most part, these spiders are pretty typical, both visually and plot-wise, although they do have the power of speech. Two things really set them apart from all other depictions, and other spider-themed monsters. First, their faces:

An utterly bizarre mix of goofy and creepy, the face seems to be an exaggeration of a real spider's, the fangs forming a beard and pedipalps become bunny ears. 

The other curious aspect is their death. Every time a spider is killed, the screen 'freezes' the image of the dead spider, fractures, and spins wildly, exactly like a kaleidoscope!

The first spider pulls an incredibly terrifying expression upon being killed. A few of the others have similar expressions, but aren't anywhere near as extreme.

You'd think this would be an "attack" face, but it's quite the opposite.

Paper Shredder

From the VeggieTales Silly Song "Where Have All The Staplers Gone?" (allegedly an obscure Broadway show tune from the unknown musical Office Supplies).

At first, it appears as just an ordinary paper shredder, one of the few office supplies that hasn't vanished.

Later on in the song, when other supplies reappear (just to sing, they aren't actually "there"), the shredder sprouts a pair of eyes though does not join in the song whatsoever. Aww...

For the number's finale, Larry grabs the shredder's waste bucket to throw the shredded paper as confetti, leaving the shredder's "head" on the floor.

Rather morbid to bringing a paper shredder to life just to behead it and toss it's innards around.

Notably, the phone and copier don't become alive as well. The shredder is seemingly the only 'real' living office supply.

Brinstar Brains

Suspended by fleshy cords in the Metroid-themed "Brinstar" stage of Super Smash Bros Melee is a gigantic brain. Being a background element, players can't interact with or even get close to it. The game itself doesn't bother explaining just what it is (despite having collectible "trophies" with colorful descriptions for many other things in it and other Nintendo games).

The Metroid series has a fair share of brains (most of which are used as computers and are hooked up to machinery), most notably Mother Brain, though this one doesn't really match any of those brains.

The acid in the "Brinstar" stage periodically rises, occasionally rising high enough to submerge the big brain's "tail," causing it to writhe in pain and sprout odd protrusions (eyes? boils? who knows).

A much smaller brain appears in another Metroid-themed stage, "Brinstar Depths." This one appears to be caged within barbed metal hoops. Again, no explanation given and no way for players to interact with it. This brain is fully encased within the stage itself, which Kraid regularly spins around (by slashing at, no less).

Curiously, both brains are utterly helpless against the stage they reside in. Other Metroid-series brains (namely Mother Brain and the Aurora Units) have complete control of their surroundings.