There are so many ways to rp monsters and undead.
Perhaps you could try to check up what people rp in the area where you play.
I remember we rped Monsters and undead to have all spawned from the sea in Madina. (which makes sense as it is surounded by waters)
Undead where the dead thrown over board as a comon burial cermony, however it was also common to throw coins in the water with them.
Those who have not given any offerings with their dead, will see the dead return.
Monsters are bred in the sea, so many monsters where rped to be somekind of sea monsters, the most scariest of all beeing the great Kraken.
In Gotland we rped the goats coming back as zombies to graze upon the skulls of their former masters.
Also other life stock or wild animals where named, at one point the capital was attacked by 20 or 30 K cs of goats and pony's.
Its fun to go along with what ever is rped in your region, makes it stronger and more fun to share.
Just a few examples.
