Zombie pigman spawning in overworld at night

a zombie pigemn without a sword spawned in the overworld, it is not hostile and acts like a normal pigman, it catches on fire temporily for around half a second then fire automatically goes out. this re-occurs every few seconds when in sunlight

i expected a zombie
instead i got a pigman

my friend found it while playing on a multiplayer public server

canoot find any particular trigger, my friend was mining and i was farming, it spawned a few hundread blaock away from me on a flat grassy surface, at nightime