Curious Coin is one of the newly currency added in Legion, which loots from Emissary caches and dungeon bosses.
Curious Coin is used to purchase items sold by vendor Xur'ios at the Antonidas Memorial in Dalaran.
Xur'ios sell some fixed recipes and other random rare goods. Recipes can be purchased at any time, but need a Curious Coin. Such as Mecha-Bond Imprint Matrix, allow Non-Goblins/Gnomes hunters to tame Mechanical pet. However, Six rare items in total, but only one refresh each day, so we can buy one item each day. The mount Arcadian War Turtle needs 150 Curious Coins. The rest five items include used items and a Toy. Mote of Light needs 50 Curious Coins. Mote of Light effect is very cool and becomes similar to Algalon in Ulduar.
In Legion, Curious Coin drops rate is very very low. It is much lower than the Legendary items added in Legion. In other words, Arcadian War Turtle will be the hardest one to get since WOW patch.
Maybe Curious Coin has been ignored by Blizzard, but it will be the potential item in future.