Was testing the custom terrain, set all sliders on page 3 as far left as possible. Heard pigs, didn't see any. Dug away the gravel where the sound was and revealed spawned pigs. Once they climbed up to 256 I could again hear them, but not see them.

Of note is that they were not suffocating in the gravel, and could not be seen even with Spectator mode's no-clipping while in the gravel.