Without seeing the art, I can think of a few explanations.

1. The artist is drawing a concept, say an epidemic/disease, as a monster to make it easier to communicate. From what I understand aincient art is often very symbilc, which would fit this very well.
2. Propaganda, the ruler wants art that shows them as powerfull, strong, and fighting/killing a sky monster is a powerfull message without angering neighbouring leaders into a real conflict.
3. Fiction/immagination, today works of fiction are very common, so why not in the past as well? Someone painted something just because it looked cool, is that so unresonable?

Aliens… /s

See also: the true terror of the mollusk horde

(Each word is a different pic)