The Tesla Model Y is believed to be an upcoming crossover version of the Model 3, akin to the Model X and the Model S on Tesla's larger platform.
In this case, the Y is expected to share a platform with the 3 and complete Tesla's S3XY lineup of electric automobiles.
It's release date has not been announced, but speculation hints at timing (or at least introduction) not too far behind the late 2017 release of the Model 3.
Renderer Remco Meulendijk took a stab at the Model Y and the result is the images seen here.
"The fourth model that will be introduced to complete the ‘S3XY’ lineup is the smaller crossover of Tesla based on the Model 3."
"The Model Y will share a platform with the Model 3 and, subsequently, be priced slightly higher. The Model Y will have a similar relation to the Model 3 as the Model X has to the Model S. As the base will be the same as the Model 3..."
Tesla Model Y Render via Remco Meulendijk at RM CarDesign