Siddhartha Gotama takes with him his royal attendant and his favorite horse. He departs out of the palace and embarks on a journey to attain Enlightenment.
This mural is depicting a being referred to by The Buddha as “Māra, the Evil One”. This is a formless being, who is only interested in causing calamity in the world. Some people refer to this being as the devil, satan, or Lucifer.
This is a real being that exists in the world, and it can influence an individual to do unwholesome things. Whether the individual does those things or not is a choice they are making however, this being can attempt to influence unwise decision making that leads to unwholesome results (i.e. unwholesome Kamma).
The last thing Māra is interested in is someone becoming a Buddha, as a Buddha represents exactly the opposite of Māra. Māra would like to influence nothing but unwholesome things to occur in the world but, a Buddha would like to create wholesomeness in the world through guiding individuals to purify the mind to attain Enlightenment.
So, this mural is a depiction from the artist where Māra is saying “No, go back. Do not go become a Buddha”, as Māra knows Siddhartha Gotama is going off on a journey to become a Buddha and influence wholesomeness in the world, the last thing Māra would like to see occur.