An alternative to the idea of ChrisselZockt without Commands.

I figured something like this must work without commands. Here is the result:

Instructions:

1. build 2 wooden stair treads (upside down) next to each other, so that they form a T.
2. on one step you build a fence and on the other a lever
3. you tie an animal to the fence with a leash and kill it.
4. build up two blocks next to the step with the lever and fix a trapdoor at the back. Then dismantle the blocks again.
5. while sneaking, build a second trapdoor from the inside to the first one, so that you have a double one.
6. you place an armor stand on the inner trapdoor, which falls down
Version: 1.9 or higher