Like a Boss.

You park your spaceship on an asteroid, in the asteroid, it has a cave.
You get out and it seems kinda damp - then it moves!
When you take off a giant worm comes out of the cave after you!

 

A giant-eyed-brain that stands in a giant jar shooting lasers at you, it can only be damaged when you shoot in its eye, and there is laser turrets that shoot laser rings on your character.

 

I'd love to see how your homeworld gets destroyed by a monster that is also going to be the final boss.
Much like alduin burned the village down in the begining of skyrim and you'll only get to fight him way later.

 

Like ants! :O
Ants are scary :C
Did you know that an estimated 25% of all animals are ants?

 

That one, is a scary statistic.

 

0.o but ants wouldn't roll up into flesh eating wrecking balls! But then again ants could be scary. But I think spiders would be even worse.

 

Have you ever seen the movie 'Antz'?

 

This would be great as a very late game boss, where you have access to some (maybe temporary) massive orbital weapons, or something like that, and have to pound away at it whilst simultaneously fighting off its minions

 

Yeah, and a potential "hardmode" (if there is one) version, would be a rebuilt mechanical version. At first it looks like the normal one, then when you begin destroying it, it turns out it is mechanical.

 

I'd like to see a Golem made from Platinum.

Weak, but worth so much money.

 

I was thinking a giant abominable snowman would be a pretty cool (Hah! See what I did there? ..) boss.

 

I wanna fight Godzilla, then if there's a hardmode like Terraria's I want to fight mechagodzilla

 

A laser shooting giant starfish that has robot tentacles. Lasers will be shot from each tip of his many arms and one HUGE beam coming from his beak! o__o its name will be Stargothicon!

 

A GIANT DUCK.
On a (slightly more) serious note, FIRE ANTS. They find you because of your technological equipment, and then you have to fight a giant one as a boss, while several other ones ruin your equipment so you have to resort to... other weapons.
Also, a giant piece of cheese that attacks you with cocktail sticks and tries to eat you.