Below is some of my raw data and here is my conclusion.

Pre-set passwords don't generate monsters. They seem to look them up in a list instead.
When you type in the password it appears that the game checks to see if it's a Pre-set and if so skips the generation routine and instead gives you a set of pre-defined stats for the monster.

Base password: Lava
Password type: Pre-set
Additional trait: up ice
Monster Sub-Type: Tiger

Zuum At6
Antlan Bo
Hornie(suezo) CJ)v?
Crevazze(dragon) D
Henger EO
Fleece(mogi)FMbx,
Karu(Zan) GX
Plesio HUaw1'
Tiger Icm.
Arrowhead KVq5
Volcano L (This is the Preset. It DOESN'T have up ice! And it's a special golem.)
Mint(pixie) NCejs8
Momo PQu3-
Gali Rf
Hare Sr(musicnote)
Monol Tdik
Joker W
PengMoch(mocchi) Y(up left corner)hz9
Ducken Z04_
Nis(gitan) (lower right corner)%
Psiroller gp+
Blue Octy(Octopee) l2
Wolfkin(Garu) n!:(circle)*
Golem y (Notice how a non-special golem with upice showed up.)
Suzurin 7
Naga "(midair dot)/(space)
Bowmew(mew) ~
Raiden =

On a semi-related note, Has anyone ever successfully mutated 99%POWER into a password that gives you something other than Stream?

Great work here, thanks!

Sorry, haven't tried anything with 99%POWER.

i'm a little confused, what is a pre-set password, are they different from regular passwords?

They are the way the game delivers Rares, or as they are known here, Specials.

Not quite correct Mr Hawley. Pre-Set passwords are the Passwords you get from in-game events. Apparently, in order to be able to provide a certain monster with an exact set of stats, they pre-programmed certain passwords into the game. When those passwords are entered it doesn't generate a monster from them, it looks it up in the table of pre-set passwords.

Excellent work, marhawkman.

This is awesome and very much appreciated by not only the younger ranchers, but this is something that my fossil-like ranching knowledge didn't know.

Thanks again.

New discovery!! Pre-set passwords DON'T have to be entered exactly to work!!!! If you add letters to the beginning or the end of the password, it will work the same.

I'm still a bit confused, but i understand it more now, thanks.