The Ares Monster is what many fans call the incarnation of Ares seen in HTLJ "Ares". This appearance occurred before the producers decided to cast Kevin Smith as Ares in XWP "The Reckoning". It could be conjectured that this monstrous appearance is Ares true form while his Kevin Smith or "Human" form is simply a way of interacting with mortals in an easier way. However Ares has never reverted back to this form in any later Hercules episodes so this conjecture would mostly be unlikely.