Witchatte

Witchatte do not initially appear to be different from the average house cat breed, with the exception of their twin tails. They're usually an inky black and have longer ears than average shorthair breeds, and most distinctively have the aforementioned double tails. However, the most interesting parts of Witchatte is below their appearance.
Uniquely, Witchatte have absolutely massive amounts of Mana, especially considering their size. This is, in fact, part of why they are named Witchatte - many witches across the realms have Witchatte for pets, due to them making excellent familiars (a creature cared for by a magic user in exchange for help with mana or specific parts of Spellcasting). They are intelligent, have an intrinsic understanding of natural magics, and have the unique ability to transfer and absorb Mana extremely quickly.
A Witchatte's most unique biology is at the base of their twin tails - twin glands allow one tail to rapidly expel Mana, while allowing the other to rapidly absorb. This allows a Witchatte to assist in a unique high-level magic technique called Stabilizing, aptly termed due to the end goal being even expelling of Mana. However, this process is complicated due to the intrinsic and chaotic nature of forming a new Spell - however, with a Witchatte, even sharp fluctuations in the caster's Mana output can be quickly offset with a Witchatte's help. Generally this process is done with the Witchatte's tail wrapped around a limb of the caster, though some very experienced caster/familiar pairs may not even need physical touch.