Carbuncle

Arcanist holding a Carbuncle

Carbuncle is a bunny/fox-esque companion summoned by Arcanist and Summoner through arcane geometry and aether manipulation.

General information

Arcanist and Summoner can spawn their pets with the actions Summon Carbuncle, Summon Ruby, Summon Topaz, and Summon Emerald. Each type has a different spell style and associated element. The three Summoning Mastery upgrades the Summoner receives between level 30 and level 45 change the Summoner's pet from a Carbuncle into various Egi. Similarly, if an Arcanist becomes a Scholar, they no longer summon a Carbuncle and instead summon a healing faerie.

Lore

“

Using geometric symbols to lend shape to living aether, certain mages can unlock the latent power within certain gemstones-a power which can be directed to the "summoning" of aetherial beings known as Carbuncles. The inherent traits displayed by these familiars differ depending on the stone used in their creation- emerald and topaz being the most common, with ruby and diamond also proving popular.

— Encyclopædia Eorzea vol 1, page 301

Carbuncle is a term for a red gemstone, such as Garnet. In this case, it refers to the gem in the head of the creature.

Proto-Carbuncle is a boss in the Pandæmonium (Endwalker) raid series.

NPCs

Alphinaud has a special Moonstone Carbuncle which heals with the same spells as Eos. He can also summon Ruby, Obsidian, and Onyx Carbuncles.
Urianger altered the geometry for creating Topaz Carbuncles so he can summon the unique Amber Carbuncle.
Tataru attempts to master the summoning of a Carbuncle in An Unexpected Ambition; an errant Carbuncle can later be captured in Wayward Carbuncle.
Ulan is accompanied by a Diamond Carbuncle which is larger than normal.
Carbuncles, with or without their Arcanist, can also be spotted in various in-game locations such as Revenant's Toll and The Gold Saucer.

Egi Glamours

Summoners and Scholars can give Egi a whole new look with the Egi Glamour text command which gives their pets the appearance of Ruby Carbuncle, Emerald Carbuncle, and Topaz Carbuncle. Egi Glamours are unlocked by completing the Feature Quest An Egi by Any Other Name (for Summoner) or A Faerie Tale Come True (for Scholar).