Pennywise The Dancing Clown

First appearance: 1990

A bunch of kids run afoul of a giant spider from outer space (possibly from another dimension) who wakes up every 27 years to feed. The space monster takes on the forms of things it's victims are scared of. This doesn't stop the kids from defeating the spider monster.
Unfortunately for the kids they didn't kill the space monster so IT comes back 27 years later to annoy the now adults to death.
90s TV Miniseries Based on a Stephen King novel and for some reason very popular even though the monster is more irritating than frightening.

Beach Ball with Claws

First appearance: Dark Star (1974)

Follow the mission of the crew of the planetary demolition space ship Dark Star.
John Carpenter's first feature film written by Dan O'Bannon (who also stars).
If it wasn't for this movie we wouldn't have Alien, Red Dwarf, or that cool stretchy star field thing we see in Star Wars when they activate their Hyper Drives.

Predator

First appearance: Predator (1987)

A group of mercenaries head to the jungles of Costa Verde to find out what happened to a presidential cabinet minister. Or so they thought! After wiping out a guerrilla camp it is revealed they were actually there just to kill a bunch of guerrillas. On their way back to the extraction point they get attacked by an invisible alien hunter who likes to skin its prey and collects skulls. Also it sees heat. Will this plucky band of mercenaries make it to the helicopter or will they just end up as more trophies on the wall of this predator's home?

For some reason, "fans" of this franchise insist on calling this species Yautja. This makes no sense to me. Who calls these things Yautja? what language is that supposed to be? It's not the Predator's language that seems to be just a series of clicks and whistles. Like the Alien in Alien, if humans are encountering these things wouldn't we just call them aliens or if we're clever predators? maybe even hunters? Fan stuff like that annoys me deeply for some reason. Predators are called Predators. Aliens are called Aliens not xenomorphs. One guy in one movie used that word to sound smart. Xenomorph means alien-form or alien-body. The marines have encountered other aliens in their missions as you can tell from their story about the Arcturans. 

Also Michael Myers should be called the Boogeyman not the shape. Through that whole series everyone calls him the boogeyman. Even his doctor. 

So remember these things are call Predators, dummy.

Also at one point the Predator was going to be played by Jean Claude Van Damme.

That would have been pretty cool, eh?

Anyway on to the trailer:

Martian

First appearance: Mars Attacks (1996)

Martians make first contact with Earth. Human's misunderstand why the martians are coming and mayhem ensues. Probably the greatest movie based on a series of collectable cards ever.

Martian saucer design is similar to the one in Earth Vs. the Flying Saucers and the Martians seem to have a similar weakness. 

Frank Douglas

First appearance: Monster-A-Go-Go! (1965)

Astronaut Frank Douglas disappears when his space capsule lands on earth. A large radioactive creature appears at roughly the same time and proceeds to wreak havoc. A team of scientists and the military track the monster in an attempt to capture it. Is this monster the missing astronaut? 

The movie was originally directed by Bill Rebane but he ran out of money before completing it. The godfather of gore H.G. Lewis eventually bought the movie in order to finish it. Between the time when the film was originally abandoned (1961) and when H.G. Lewis took over (1965) enough time had passed that some of the original actors were no longer available so they are replaced by different characters providing similar roles in the plot. One of the original actors had changed so much in the four years he was recast as the brother of the character he originally played.

this film also features a human voice simulating the ringing of a phone at one point.

Green Slime

First appearance: The Green Slime (1965)

Scientist visit an asteroid on a collision course with Earth. After blowing up the asteroid they return to the space station Gamma 3. Unfortunately one of the scientists has a big of green slime on one of his boots. The slime grows into a weird tentacled creature that can shoot lightning out of its tentacles. When ever it's wounded it bleeds the same green goop that it grew from creating more monsters. Can the crew of the Gamma 3 stop this menace before it takes over?
Probably not the seem more concerned about their personal lives and junk.
Features probably the most awesome sci-fi movie theme song ever.


also here's the trailer

The Blob

First appearance: The Blob (1958)

One night a strange rock falls from space. A weird old hermit decides to poke it with a stick. Inside the rock is a weird pulsating ball of goo. Naturally the old man pokes the goo with a stick. The goo, being hungry from its long space voyage, climbs up the stick and proceeds to begin eating the old man's hand.
The hermit stumbles out onto a near by road and proceeds to ruin Steve Andrews' (Steve McQueen in his debut leading man role) evening.
Based on a true story.
Features a theme song written by Burt Bacacharach

Killer Klown from Outer Space

First appearance: Killer Klowns from Outer Space (1988)

Late one night a strange circus tent shaped spaceship lands on the outskirts of town. The occupants proceed to harvest the people of the town. It's up to two horny teens, their best pal, his girlfriend and their buddy in the police force to save the day! You won't look at cotton candy or popcorn the same again.

Much more terrifying clowns than Pennywise could ever hope to be!

Features a fantastic theme song by the Dickies

The video for the theme song:

Crite

First appearance: Critters (1986)

Small ravenous alien criminal carnivores steal a spaceship and crash in a small rural american town making life miserable for the family they land near. Luckily bounty hunters UGG and LEE are on the case! The town drunk gets a new lease on life.

One of the better "little monster" movies to come out in the "let's make another Gremilns" era of filmmaking. Spawned 3 sequels (two more than Gremlins) and started the tradition of "We've got to set one chapter of our franchise in space" usually Part 4.