The design of the rock changed on Jan 13th, 2017. The design was created by a person named Shaun Goodwin.
However, due to negative feedback, the design of the rock was changed back on Jan 17th, 2017. The Design of rocks are kept in the files of the game. http://mope.io/img/rockhill.png, while you can still have the old design of rocks in the Teddy Bear Mod Pack.
In the prior of December 28, dragons were only one animal that can go trought rocks.
Rocks are found in every biome, mostly used in the Arctic