Final answer:
Gustave Moreau is the artist who painted "Diomedes Devoured by his Horses", which reflects his symbolist style rooted in mythological storytelling.
Step-by-step explanation:
The artist behind the painting "Diomedes Devoured by his Horses" is Gustave Moreau. This subject is rooted in the arts, specifically focusing on the creations of historical painters. Gustave Moreau was known for his classical, symbolist approach to art which often intertwined with mythological themes, as seen in this particular painting which draws from ancient Greek mythology.