everything in the warhammer game is a unit. even single characters with US 1 are a unit that follow rulse for skirmishers, thus they have flank. the only units i am aware of that does not hae a flank/rear is the varghulf wich has a special rule for that.
regarding the stegadon, if you look up the BRB for monster's line of sight it is mentiend there that the line of sight is used to determine what the monster can charge and its flanks/rear so the stegadon can be flank/rear charged as normal.