SIQ

SIQ is some kind of drug developed by the Golden Lion Pirates scientist Indigo, to make animals "evolve" and develop special abilities (i.e. giant size, mutation). Shiki used this to build his army of mutated animals for twenty years, in the hopes to overthrow the world. The Straw Hat Pirates eventually destroyed the entire stock when the bombed Shiki's ship.

Origin
This substance is derived from a natural substance called IQ, only known in Merveille. The IQ originally makes the animals evolve adapting to their habtat. As Indigo developed the SIQ, the more he injects this substance into animals, the more they developed their angry nature.