A monster is any creature that is in part an influence from the infestation. Creatures that have evolved on Shashnia but can be aggressive to myn are considered 'Animal', and should be tagged as thus.
The magic rich area has caused these creatures to try to invade. They are just trying to eke out an existence as myn are. Unfortunately, they do so in a destructive and infective manner.
Conquering galaxies is all well and good for a species, but some are too greedy and nasty. These are the fellas that we call monsters.
Luckily, monsters can fight against one another just as much as with the myn. The lack of organization keeps them from being to dangerous.
There are villian characters that have sizeable armies and treaties that can cause some havoc.
Other monsters are sent as scouts out into the main world; to try to find a place where a horde can invade.
Original Monsters with pages in the encyclopedia
http://members.dsl-only.net/~bing/frp/fudge/cvt/srd_monst_index_name.html - Conversion of D&D OGL monsters to Fudge
Other OGL projects are a good place to get inspiration from, but monsters need to have Rank stats converted to use our mechanics, and should be attributed
http://www.d20openrpg.com/gamemastering/challenges/monster-listings
http://www.d20pfsrd.com/bestiary/rules-for-monsters/creature-types
http://en.wikipedia.org/wiki/Category:Mythic_animals
http://en.wikipedia.org/wiki/Category:Legendary_creature_stubs
http://en.wikipedia.org/wiki/Category:Mythological_plants
http://en.wikipedia.org/wiki/Cryptobotany
http://en.wikipedia.org/wiki/List_of_cryptids