This toy is a reward from It's In The Mix and related to the Pool of Mixed Monstrosities in House of Plagues, south of the Maldraxxus Plague Watch windrider. You need to summon 7 rares to get the toy.
Here you can buy Smelly Jelly Toy Boost for EU or US Servers.
You'll get guide for completing Smelly Jelly Toy Boost
