Pokémon has had its fair share of legendary Pokémon since Generation 1, with Mewtwo being the eternal fan-favorite. Legendary Pokémon have, for the most part, all had a series of typical characteristics in common -- and then came Type: Null.
What is Type: Null?
Type: Null is a Pokémon introduced in Generation VII. Wearing a mask to control its powerful abilities, Type: Null was created by Faba of the Aether Foundation. Its purpose was to be an "Anti-UB Fighting life-form." Because of this, it received the code name "Beast Killer" and was developed with cells taken from all known Pokémon types.
In that sense, Type: Null bears resemblance to Mewtwo, which was artificially created by humans. And in the same way that Mewtwo was meant to be a recreation of Mew, Type: Null was meant to be a recreation of Arceus, the Poké "God." The idea was to have Type: Null be capable of switching between types -- provided it possessed the right memory. This was inspired by some of the myths researched at the Canalave Library.
The Pokémon's original name was Type: Full. Three models were created, but due to all of them rejecting the RKS System, they all became out of control. After getting the creatures back under control, the Aether Foundation gave these three Pokémon limiter helmets in order to lessen the effects of the RKS System which they so heavily rejected. The Pokémon were put into permanent cryogenic stasis.
Following these events, the Beast Killer Project was considered to be a failure, and the newly created Pokémon's name was changed to Type: Null. Before the events that occur in Pokémon Sun and Moon, Gladion was able to free one of the Type: Null, after which he chose to adopt it. His hope was to find a way to undo the effects of the limiter helmet in order to allow it to use the RKS System.
In Pokémon Sword and Shield, a new Type: Null appears in the Galar region. It was apparently created by Macro Cosmos through research notes stolen from Alola.
How Legendary Pokémon Have Changed
Type: Null has quite a few things that set it apart from other Legendaries, causing many fans to debate its "Legendary Pokémon" status.
One of these things is the fact that Type: Null, unlike other Legendaries, is capable of evolving. Once it finds a partner it trusts, it destroys its own control mask and evolves into Silvally. After becoming Sylvally, Type: Null reverts to its original wild temperament. Despite this, it will remain loyal to its Trainer, even at the cost of its own life if necessary. The destruction of Type: Null's mask helps re-activate its RKS System. This allows it to change its type according to the memory it's given.
Another thing that sets Type: Null apart is that, unlike most Legendaries, it is not unique. There are three different Type: Null specimens that were created by the Aether Foundation. Its catch rate is thus technically higher than that of other Legendary Pokémon.
Lastly, Type: Null is the first Legendary Pokémon to be gifted to the player by an NPC.
What This Means for Type: Null
Despite these differences, Type: Null remains similar enough to other Legendaries to be classified as one. For instance, it is genderless, just like all other Legendaries. It also remains extremely rare and does not exist in the wild. Additionally, it has the highest base stat total of all Pokémon capable of evolving and cannot be bred with Ditto. The official Pokémon website also referred to Type: Null as a Legendary in a 2018 Pokémon Quiz 2018.
Although Type: Null is a bit of an oddball addition to the family, there is no doubt that it remains a Legendary Pokémon -- even if its stats might not be the best. The question, however, will most likely still be debated by fans for some time to come.