Visual illusions where you experience two equally possible interchangeable states in perception are called “bistable illusions“. The Necker cube and the Rubin vase are ones of the most classic examples of a bistable figure. I have discovered that all over India, you may see many variants of an interesting bistable depiction, which represents a bull and an elephant with distinct bodies and only one head (painted or sculpted at least 850 years ago). If you look carefully at the whole picture, you may see how the body parts of both animals are skillfully overlapped. For instance, the trunk of the elephant is also the hump of the bull. The horns and ears of the bull have become the mouth or snout of the elephant. These are ones of the earliest documented “ambiguous figure” illusions…
Source: Text and pictures taken from my book “L’étrange univers des illusions d’optique“.