I dunno, I find it really sticks out that several single-typed Pokemon in particular have that honorary second type in the form of their movepools and attributes, but don't actually *have* that type. I wonder what exactly went into the design process for that kind of Pokemon, and whether there's a design standpoint explanation for those instances or if there's an in-universe reason they would only be single-typed and not have a second type.
One that really stands out to me is Psyduck. All its lore, all its Pokédex entries, and all its Anime appearances (of which there were lots) focuses on its psychokinetic powers. Heck, it's even in its
name. It learns the usual suite of Psychic-type moves. Yet it's not Psychic-type.
I can understand why they chose to retain a pure Water-typing for it back in the Gen I days, as there were two Water/Psychic lines already (that being said, if given the choice I'd have made the Slowpoke line pure Water instead, as its lore is more centered around its slowness and carefree nature than its psychic powers). However, as you say, "honorary typings" are a thing and a Pokémon doesn't necessarily need to have a typing to exhibit the traits of one. New mechanics have given plenty of opportunities to showcase Psyduck's abilities. But despite Psyduck's frequent appearances in all Pokémon media and its psychic powers being the whole focus point of its lore, very little has been done to showcase them in the games.
For instance, you would think its headaches and psychic powers would be the perfect base for an Ability. Arguably, one could say Psyduck's innate psychic powers was the first description of abilities in Pokémon, years before they were formally introduced. But none of Psyduck's abilities have anything to do with them. There's Cloud Nine, whose Japanese name can mean "care free", which reflects on Psyduck's derpy and oblivious attitude. It also has Damp, which is a generic Water-type ability. Not a single mention of headaches that unleash its psychokinetic powers. Even the
Detective Pikachu movie got that thing right. When Gen V came along with Hidden Abilities and another opportunity to give Psyduck the headache it deserved, it instead got ... Swift Swim.
More generations, more opportunities. But there was no Golduck evolution in Gen IV. No Mega Golduck in Gen VI. No Psyduckium-Z in Gen VII. No Alolan form, no Galarian fom. No signature move. All Psyduck has is that suite of widely distributed Psychic-type moves, none of which have any explicit connections to its headache problem. It's so strange that such a prominent Pokémon - even the favourite Pokémon of Junichi Masuda, reportedly - is so famous for a trait the games don't seem to showcase in the slightest.