What does Ferocious Fang do?

You do not have to hunt for one: a free Ferocious Fang comes with your starter, and a golden version turns up in the daily login bonus. Most players have one before they have made any real choices about their team, so the question is which Aniimo gets it, not where to find it.

Give it to whatever you rely on for damage. Its bonus is to attack, so it is wasted on a healer or a support. Those Aniimo want something like Miraculous Fleece instead, which increases healing done by 5% at its listed tier. Because the Fang counts as a general all-round item rather than one tied to a narrow role, it suits almost any attacker you put it on.

Stats on their own, or a skill as well

Equipped without a contract, the Fang still gives its attack bonus but no skill. With an activated exclusive contract it also enhances the holder's core active skill. Alphas get more freedom here: an Alpha can equip any held item, including one already linked to another Aniimo, and keep its enhanced core skill.

For a normal, non-Alpha Aniimo, how the Core Skill gets upgraded is not settled. A contract with a Legendary Held Item is one of the routes described, and the Fang is legendary, so it may matter for that too.

Runes no longer apply

In the closed beta, runes were socketed into held items to build up orbs and unlock tiered effects. Pawprint Studio removed the rune system entirely at the 16 September launch, so you cannot socket anything into your Fang, and beta builds that did are out of date.

Not settled: How far the Fang's legendary rarity counts toward upgrading a non-Alpha Aniimo's Core Skill is not settled, because the upgrade route itself is still unclear.