Maybe you want to use this plugin: getandmob. This way your script will be easier to make.
Most basic sample:
map,x,y,dir script MonsterHunting sprite,{
OnPCLoginEvent: // So that newly logged players can't exploit
if ( lastdate != gettime(8) ) { // gettime(8) returns day of the year. Error margin: if someone logs in again after an exact year.
set mobID, getrandmob(0,99);
set lastdate, gettime(8);
}
dispbottom "Your mob of the day is: " + strmobinfo(1,mobID);
end
}
I assume you know basic scripting for doing the rest of what you want. This, as requested, just picks you a mob that can be different to another player's.
