this is an ideas specifically for gwis rather than a general mob.
the pk-ing express is a money in transit affair hence the express
what this would consist of is a group of heavily armed mounted riders crossing a larger un-disclosed pk zone like thelinor, TD etc
once they cross to the other side of the map thats it 'invasion' over
the gwis would just announce its deploy something like "the pony express has entered bandit country"
It would be preferable if the wagon art in game could be used as a mobile unit but i doubt that is possible and if it is probably far to time consuming to happen.
so the heavily armed mounties that for this unit would need to have guild bot like builds so they not an easy picking, ( can the server get them to travel as a tight group across a map ?)
specifically deployed in pk maps only this would bring out the baddest of the bad characters we have in el to attcak the pony express and risk themselves being pk'ed.
drops would be a high gold coin amount and metal bars ranging from lowest to highest, this could be a random mix or all of the same, tin, bronze, gold, hydro etc
or maybe if the value needs to be bumped up to make it worth while just hydro bars and gc?
maybe they would also have high grade armor and weapons drops as an option too?
the reason for suggesting this as a giws affair is to keep it rarer, not to be a daily hunted to death mob.