There are numerous murtis (idols), sculptures and paintings of Hanuman stamping on demons. One of the most famous painting and sculpture of Hanuman standing on demon is that of him tramping on panvati (bad astrological influence). Others demons are identified as Ravana, Kumbhakarna, Indrajit (Meghnad), Kalanemi and Mahiravana.
It is believed by most Hanuman devotees that Hanuman had on one or more occasions defeated the above said demons.
Panvati, or Panoti, is a demon mentioned in Puranas and she symbolically represents the all the bad astrological influences caused by the Navgrahas or nine planets. The most popular image of Panvati is that of Hanuman trampling on her. This means that Hanuman can remove all the bad astrological influences.
Mahiravana who had kidnapped Sri Ram and Lakshman was defeated by Hanuman. (You can read the story here).
Kalanemi was defeated by Hanuman when he was on his way to get Mrita Sanjeevani. (You can read the story here).