The new mobs introduced in the Nether Update, the Strider, Piglin, Piglin Brute, Hoglin, and Zoglin, don't have a non "minecraft:" identifier in the command menus. All other mobs (i.e. Pig, Cow, Ender Dragon, Blaze, etc.) have a "minecraft:" identifier (minecraft:pig) and a non "minecraft:" identifier (pig). But for some reason the new ones don't have this. It's all "minecraft:piglin", "minecraft:hoglin", "minecraft:strider", "minecraft:zoglin", "minecraft:piglin_brute".
This makes command work much more difficult as it requires more typing and can confuse some players when they can't find the mob they're looking for because the non "minecraft:" identifier one.