In the Osai region there exists a phenomenon called Infusion. In ancient times, offerings of precious stones were made to Cerasormr and Fulguroc, and over time some of their unique power was absorbed by these stones. Many of them have since been given to humans and pokemon the guardians of the land deemed worthy, while others were stolen from caches, or have become lost. Infusion stones glow with a dim light, hinting at the power contained within. Any pokemon can use Infusion to cloak itself in an aura of energy, raising the power of its Primary typeand giving those Moves an additional seondary effect (stat down, burn, etc) for the duration of combat. A few pokemon react to the energy more strongly and change their form and even gain a different Secondary type and/or Ability. These are two examples:

Pikachu + Infused diode
Infused pikachu, or "pikablu" as its fans like to refer to it, is the form a pikachu can take while it holds an Infused diode. It takes on Water as a secondary Type and gains the ability Conductivity: Water type attacks have a small chance to paralyze an opponent. Its appearance is very similar to marill, an unrelated rodent pokemon from the Johto region, and the diode is obviously man-made and of recent construction; many wonder who created it and why.

Raichu + Infused Dendrite
Infused raichu, or "gorochu" as its fans like to refer to it, is the form a raichu can take while it holds a piece of Infused dendrite. It does not gain an additional Type but its ability becomes Rolling Thunder: consecutive uses of Electric moves by this pokemon cause its Special Attack to rise. In this state is it wildly aggressive and seems to revel in its destructive power, though fortunately this means that protracted battles with this pokemon are rare, so it reverts to its normal form quickly. An effort in Kanto was made several decades ago to breed a raichu that could become "gorochu" permanently but the experiments ended in failure.

(also you can totally find Mew under a truck somewhere in Osai)