This Slayers! t-shirt can be found in various stores, from various designers and sometimes with changes. The most common change is the word “Slayers!” sometimes says “Buffy”. Trademark issue?
Of course, what we have here is a Scooby Gang mashup with the Scooby Gang.
This design is on sale at Redubble from Nik Holmes. The style we see in this t-shirt is similar to that of his either designs.