Hello! This is a series of blogs showing you how to make custom mobs, in tutorial #1, we are going to make space invader aliens in Minecraft!
Step 1: Get a command block: /give @p command_block
Step 2:Paste the following command into the command block:
/summon Spider ~ ~1 ~ {CustomName:"Alien",CustomNameVisible:1,ActiveEffects:[{Id:14,Amplifier:1,Duration:2000000000,ShowParticles:0b}]}
I'll try to explain:
/summon Spider ~ ~1 ~ - This part summons the spider 1 block above the command block (using ~ ~1 ~)
{ - Means there is an NBT tag
CustomName:"Alien" - Makes the spider have a custom name (Alien)
, - An NBT tag seperator
CustomNameVisible:1 - No need to pay attention to that :/
ActiveEffects:[{Id:14,Amplifier:1,Duration:2000000000,ShowParticles:0b}] - Makes the spider have invisibility (id:14) with a level (Amplifier) of 1 with a duration of 200000000 ticks, ShowParticles:0b means that it does not show the effect particles (those little swirls)
Now make a redstone clock that is hooked-up to a command block:
[ ]= Command Block
{ }= Command Block with redstone on top
*= Redstone dust
v=Repeater (pointing to pointed direction)

*{ }
^v
*[ ]

Set the repeaters both to 3 ticks (right click 2 times)

Paste this command into the command clock with redstone on top:
/entitydata @e[name=Alien] {CustomName:Dinnerbone}
Paste this into the other one:
/entitydata @e[name=Dinnerbone] {CustomName:Alien}
What this will do:
Rapidly flip the mob up-side-down and "down-side-down"
How this works:
When naming a mob "Dinnerbone", it will flip up-side-down, in this clock, we rapidly change the name to Dinnerbone then Alien the Dinnerbone then Alien then...
Now activate the clock!
Activate the command block with the /summon command and you should get a space-invaders alien!
It's a bit invisible though, if anybody knows how to fix it, please tell me by comment, thanks for reading!
please diamond, too. 

*replace Spider in summon command with CaveSpider to get a smaller version!