Where Did The Mythological Creatures Go?

Some works of fantasy are based on the premises that mythological creatures existed, as we can read in Homer etc, but do not exist now, as we can see. So where did they go?

Neil Gaiman presents a brilliant answer about what happened to the Fabulous Age of Baghdad and CS Lewis has a poem about how the unicorn did not make it into the Ark but I am here focusing on three stories by Karen Anderson.

"Landscape With Sphinxes"
After the glaciers melted, sphinx-headed bipeds, which might be game, wandered in...

"Treaty In Tartessos"
Agricultural men were unable to coexist peacefully with centaurs who needed vast, uncultivated areas in which to hunt, so the men let the centaurs migrate to the newly discovered land of Atlantis...

"The Piebald Hippogriff"
The first two stories are sequential: hunting men eliminate sphinxes, then agricultural men displace centaurs. However, the third story locates hippogriffs in an entirely different universe where the world has a fenced off edge, space is blue and a hippogriff can fly to the moon.