My girls recently started getting into Playmobil sets and have several with horses. They wanted horse corrals to go with them. Seemed like a fun thing to design and 3D print. The sections just snap together and can rotate to make corners. They seem pretty sturdy. The kids haven't broken any yet.
[Update 2016-09-17]: Added a gate piece with a door and a latch. The latch uses two M2x8mm cap head screws.
[Update 2018-03-06]: Added a modified version of the fence with rounded joints that allows for free rotation. This allows for corrals to be shapes other than square or rectangle. Thanks for the suggestion @Coudys.