Epics are apparently evolved Rouges.
It would go in this order:
Rogue(Mini-Epic based off the Prima game guide)->Half Epic->Epic->Hyper-Epic(?)
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Half epics are very, and I mean VERY rare, or simply a programming glitch. They don't have the behavior of a normal epic, but are (possibly) the following:
They may be large rogue creatures. Half epics can be befriended to sing and fight in your pack. Killing them does not count for the achievment. ~Spore Wiki
Wiki page:
http://spore.wikia.com/wiki/Half-Epic_____________________________
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Hyper epics never have an appearence (Alive), but appear in bones. You can create a hyper epic by using T-Search or any hacking software that allows you to break the resize limit in the Adventure creator. (Refer to this:
http://www.youtube.com/watch?v=qpGFWkP6CCE ) They may also leave footprints. It is beleived that you can see a Hyper Epic through space, however, due to their lack of appearences Ingame, this is unproven.
People beleive that the Sea Monster was a Hyper Epic before evolving into a Sea Creature:
Footprints that could of been left by a hyper-epic sometimes can be seen leading into an ocean which could mean that they evolved into sea creatures, and from there, the sea monster we all know and (dont) love, though it is highly unlikely due to thier size~Spore Wiki
Size Comparison:

(Picture taken from the wiki)
Here, you can see a normal creature, a normal epic and the bones of a hyper epic, they are really huge.
Wiki Page:
http://spore.wikia.com/wiki/Hyper_Epic_Creature