And here my ideas for the monsters (
http://www.hostingpics.net/viewer.php?id=824593magivalg.jpg I make some idea of places where the monsters can lives)
For the Beginning, the Tarvos (aka the centaur ):
In my opinion, the Tarvos will lives in the east of jehanna, a semi-desertic regions near to Lagdou ruins, a forgotten great city/temple at the glory of Fomortiis who became a ruin. The Centaur will be leads by a great Khan (we can imagine that there are nomad or tribal) and Follows the cult of Fomortiis (I will talk about this later). The Centaur can have children like humans and also have child with a human... But you doesn't want to know how
(Also a child from a inter-species marriage will have the trait "half-centaur" with some bonus but also a malus of opinions). It's a monster's species who can became a great enemy for humanity... Or a great friend if they abandon there unholy faith. They have the life expectancy of humans. A great Tarvos warlord will have the trait "Malduin" who gives him some bonus
The Gorgons:
The gorgons will lives in the Mt. Neleras and some little forest between Jehanna and Rausten. They will have access to dark magic and to have descendant, they will have the possibility to "spawn an egg" One time per year, and in fact it's decision will spawn a gorgon's daughter. And the gorgons will have the possibility to petrified there prisoners. Also gorgons have a short life, but they can make human's sacrifice to change that
The Cyclops:
Cyclops are "bless" with a very long life, for centuary and centuary, and a great strength but not a great mind... And they are only represented by some little tribes in the dark forest of the east of Rausten. No one know how Cyclops procreates but cyclop's warchief adopt some of them soldiers in there "dynasties". They can also eat humans if they take prisoners. And no one want to face a Cyclops on the battlefield.
The Bael:
By the chapter 6 of FE8, we know that some bael lives in the south forest between Grado and Renais, so this places can be a great nest of Baels.Also, Baels have a very long life and like the gorgon they can spawn some little creepy spiders (but only one time per ten years), and eat prisoners. I think that Bael and Cyclops needs also special units to truly represent them.
The Moggal:
Nothing new, dark magic like Gorgons and they will have the possibility to spawn a new comrade with the sacrifice of a filthy human.
They will lives in the little Island near to Grado. And a Moggal who makes many sacrifice and enhanced his magic power can become an Arch Mogall. A powerful magic creatures
The Gargoyle:
They will live on the others island, and will have the possibility to adopt others gargoyle in there dynasties, because Gargoyle don't procreate, they are basically stone.
A strong Gargoyle came become a Deathgoyle
And for Fomortiis, his cult and some others features I will need more times