The miner is a monster added in the Convict patch. Miners are tunneling monsters that need an item of the pick-axe class to work. They are never randomly generated, and only appear in the Convict quest.
Eight miners are generated on each lower filler level of the quest. When you first enter these levels, you get the message "This appears to be a prison level that is still under construction", making the miners the construction crews that are excavating the new levels.
The miners are generated peaceful, and do not seem to care about the escaped convict wandering through their workplace.
Reference: Karl Garrison's Convict Patch