Zoglins aren't considered an undead mob, even tho Piglins are.
Consequences:
unaffected by smite
attacked by the wither
the wither is hurt by the zoglin
not healed by harming and poison potions/harmed by healing potions
can drown underwater
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Also note that zoglins are like all nether mobs (except for hoglins and piglins), they are immune to fire.

They will attack squids and then drown

Doesn't seem like it has been fixed
Also note that zoglins can harm the wither, even though the wither is supposed to be immune to all undead mobs.