Felyne are a member of the Lynian class, which consist of monsters that resemble cats. Felyne are bipedal and are capable of human language, which allows them to enter a human village and assimilate themselves into the community. Felyne take on a variety of jobs in the village, such as a chef that cooks food for hungry citizens, as well as joining hunters on their journeys as a companion; Felynes who partner with a hunter are known as Palicoes and wear miniature suits of armor.
A specific Felyne with a more cartoonish design, the one in which the Mii headgear is based, is the main character of the Felyne Village series of spin-offs.
As a costume
Felyne was announced as a Mii Fighter hat on March 4th, 2021, and was released later that day, alongside Pyra and Mythra. In its reveal trailer, the Felyne Mii can be seen throwing a Boomerang and using Shot Put, referring to the throwables that they will toss at monsters to aid the hunter during quests. Additionally, the Felyne Mii is seen fighting a blue Banjo & Kazooie referencing Arzuros, a large bear-like Fanged Beast originating in Monster Hunter Portable 3rd. They are also seen running from the Peckish Aristocrab, which references the Daimyo Hermitaur, a large crab-like monster known as a Carapaceon introduced in Monster Hunter 2.
Names in other languages
Article on Monster Hunter Wiki