Dude,regular monsters weren´t possessed.Only on special occasions.
Yeah,they are so clever that they went to winhill only when Ellone is persecuted.
Papparazi monsters?
Or they just appears to acquire a human form(like double president and that monster on the Esthar highway).
And they just happened to concentrate in one place on the moon?
The game tells us that it is the influence of the crystal pillar.Although I don´t tottaly believe that ,it makes more sense than to say it was monster nature.

Really,you think that Deling make an agreement with gerogero to change as the presifdent.

Deling-Would you help me here?
Gerogero-Bwaaaaarrgh
I only need you to appear as me to deceive the resistance.
Gerogero-Gruhaaaa..Bwarrrrrgh.Bwarg.

Really.It makes a loooooooot more sense to say that the monster get possessed by the same person who possesses Deling or at least by Deling (if he is not possessed).
These were the only two hypotheses.

There is much more in game evidence to support this than to support the opposite.
Not only Gerogero has a resemblance with a shumi,but we know that arms grow when they(Shumis) change the environment and they became what is on their hearts.