Birds and Ponies aka "Horses and Birds"

Wood engraving
 
This is one of only three small prints (Fishes and Frogs and Fish and Boats) in which the background of one image undergoes a metamophosis to become a second image that tessellates with the first. In essence, they are miniature versions of Sky and Water. Horses and Birds was utilized for three different Escher exhibition invitations as well as in the print Metamorphosis III. Fishes and Frogs was used for one exhibition invitation as well as the lower aspect of the print Verbum.  
 
Horses and Birds is the rarest of the three.   
It is available.