The Phantom Thieves of Heart are receiving new abilities and powers in Persona 5: The Royal, and Joker is no exception. It has been revealed that Joker will be able to summon a creature that has not been part of the Persona series since Persona 2: Innocent Sin and Persona 2: Eternal Punishment in the form of Fafnir the dragon.
Like almost every other creature in the Persona/Shin Megami Tensei series, Fafnir is based on a concept from mythology. Fafnir comes from Norse mythology. He started out as a dwarf who was transformed into a dragon due to a curse. The legend of Fafnir would go on to inspire the creation of Smaug in The Hobbit. Fafnir has appeared as a dragon in many games in the Shin Megami Tensei series, although a more refined design was introduced in Devil Summoner: Soul Hackers.
The last time Fafnir appeared in the Persona series was in the Persona 2 duology, in which he had a traditional dragon form. A new video has been uploaded to the official Persona 5 Twitter page (via Siliconera), which shows Fafnir being created as a Persona that Joker can use in Persona 5: The Royal.
The version of Fafnir that will appear in Persona 5: The Royal will use its updated design from Devil Summoner: Soul Hackers. This isn't the first time that Fafnir has appeared in a 3D Persona/Shin Megami Tensei game, as it also appeared in Shin Megami Tensei IMAGINE, which was an MMORPG that was released in 2007.
The trailers for Persona 5: The Royal have mostly focused on the upgrades that Joker's friends will be receiving in the game, including team-attacks and brand new Personas that can be used in battle. The fact that Joker is a Wild Card means that he has access to more Personas than any of his teammates combined, so it's possible that the developers felt that he would only need access to some new Personas in order to upgrade his abilities.
Persona 5 Royal is due to be released on PlayStation 4 in Japan on October 31, 2019, while Persona 5: The Royal will be released on PlayStation 4 internationally in spring 2020.
Source: Siliconera, Persona 5 Twitter