Loach Pearl is a very rare resource in Genshin Impact that can only be gotten from Golden Loach, a type of lizard that can be found in Liyue. You will normally only find one of these lizards out in the wild in any given location, but they can appear in groups as well, so prepare to be quick on your feet.
On the images below you will see various area maps, with the Loach Pearl location shown by the player indicator. You will need to be quick, as these guys can move quite fast. Just grab them as you would any other animal or flower in the game.
They can normally be found right on the edge of the water in the Stone Gate/Dihua Marsh/Wangshu area of Liyue, so you may need to swim after them. The Loach Pearl is currently not used for anything in the game but is thought to be an Ascension Material for a future character.
As we discover more location, we will update this article.
Stone Gate #1
Stone Gate #2
Dihua Marsh #1
You can actually grab three of them in the same spot here if you are quick enough.
Dihua Marsh #2
Dihua Marsh #3
You can find three of them south of the bridge, blow the Dihua Marsh Statue of the Seven