Minecraft wiki entry for Nether_portal, associated text: Zombified piglins have a chance to spawn on the bottom frame of the portal in the Overworld in Java Edition if any nether portal block above receives a block tick. In Bedrock Edition they spawn in certain squares adjacent to the portals in the Overworld, not inside them. Zombified piglins spawned in this way have a full 15-second portal cooldown, meaning they can't go through the portal they are spawned in unless they leave the portal for a while. They spawn twice as often on Normal difficulty as on Easy, and three times as often on Hard difficulty as on Easy. No other mobs can be spawned by nether portals in this way, in any dimension.