Can YOU spot what's inside the red circle? Optical illusion sight test with a hidden image leaves the internet baffled

This optical illusion hidden inside a red dot (left) has left users scratching their heads. Some can see a detailed image or just an outline, while others see nothing at all. 

When the dot is flipped you can clearly see a detailed sketch of a horse - complete with a mane and tail, saddle and bridle and grass around its feet (right). 

The image is said to test people's vision and is the latest in a string of a popular brain teasers sweeping the net.