First appeared in Galaxy. Reprinted by H. L. Gold, and Groff Conklin, and the radio program, X-Minus One. If X-Minus One did a program on it, you can trust it will be fun if not thought-provoking: Audio dramatization.
In 2013, Wyman Guin was awarded the Cordwainer Smith Rediscovery Award "to honour notable sf and fantasy authors who in the view of the judging panel did not receive or no longer receive the attention they deserve." --SFE
A man genetically creates new creatures, which he hides from his family. He wants to create a grand practical joke where he fools not only the creatures but also, primarily, the humans into thinking a race of being existed on Earth long before the humans. However, his kids find the creatures and play with them, and his wife spills the beans about this being a joke. The creatures learn of this but don't hold it against him. Instead, they hitch a ride to Mars, and the protagonist wants to join them.
Much of the character interaction is reminiscent of Nancy Kress's work. However, she would unlikely have left it as a practical joke. The story might have gained significance had it supplied a motivation for the joke, i.e. "The scientific establishment/Communist pinkos/Capitalist pigs won't laugh at me anymore. We'll see who gets the last laugh." or whatever to infuse his actions with more meaning than an elaborate joke.
Edition: Beyond Bedlam and Other Stories by Wyman Guin