Is there any mythical animal that symbolizes riches wealth?

Dragons spring to mind... But a creature can mean what you want it to mean...

 

Well, not in fantasy that I know of, but there is the Pixiu in Chinese mythology. A winged lion/lioness.

But creating your own mythology/folklore could be fun too!

For other inspirations, I'd suggest looking at Hindu associations with Lakshmi (associated with fortune and who is often associated with owls) and Kubera (wealth).

Or perhaps the way Jambhala, in Tibetan lore, is a five faceted Deity of Wealth and has a color association as well as both an element and a horoscope animal(s) attached to each of the five incarnations.

And Tsai Shen Yeh is a Chinese god of wealth, often called Lu Shing or The Star God of Wealth. Carved figures of this deity are often left near entrances of building and homes to invite wealth.

From these myths and folktales you could transfer similar aspects to an animal/creature in your world just as easily.

I've borrowed from this particular idea for a story I'm writing. I have very surly looking cat-like creatures, called Grimalkins, who are the familiars of one of my fantasy races. They aren't related to wealth in my story but the point I wanted to make was the Grimalkins never actually appear, except for these little carved figurines of them that pop up, left by unseen hands, and people believe in them. This was also inspired by the fact that once, in an apartment building I lived in, a troll (the cute little 2" tall toy with crazy hair) was left on the cross piece of the door frame over my door one night. Never found out who left it there but I did not remove it til I moved out a few years later. Moving there had already been really good for me so. . . Superstition!

Even if the animal exists in your world, to have it be a perceived as being only a myth or folktale can be an effective way of introducing / foreshadowing it's presence in your story ahead of time.