What I expected to happen was...:
I added a file called pig.json with the default vanilla content. I created a world with Experimental Gameplay enabled and attempted to spawn a Pig with its spawn egg. I expected a pig to spawn.
 
What actually happened was...:
Instead, the pig didn't spawn, but the other entities that were already spawned in the game froze, and although I could still move, whenever I kept hitting the ground with the Pig spawn egg, no pigs spawned.
 
Steps to Reproduce:
1. Create a behavior pack manifest and a folder called "entities".
2. Create a file called "pig.json" inside the "entities" folder you just created.
3. Copy-paste the default "pig.json" content from the Vanilla files.
4. Pack the behavior pack and load it into the game.
5. Create a world with Experimental Gameplay enabled.
6. Select a Pig spawn egg and right click the ground.