Are all Pokemon capable of appearing in Hordes?

Question

Or is it only limited to some Pokemon?

Additionally, how do you tell if a Pokemon in a Horde has its Hidden Ability?

Answer 1

It is not all Pokémon that appear in hordes.. On Serebii they have really good coverage
This is hordes on Route 10 if you scroll down

• You don't know if a Pokémon has it's Hidden Ability, only if it shows in battle. For example if you want an Yanma with Frisk on Route 10, you just have to make sure your Pokémon holds an item. If you want a Nosepass it's a bit harder to see it's Sand Force because you can only guess if it deals more damage...

Maybe there's an item or ability capable of checking it's ability, I don't know