From the Super Mario Wiki
Toad's cousin is a turban-wearing Mushroom boy, although Toad has at least two other cousins, whose names are known: The Mushroom Marauder and Jake "The Crusher" Fungi. Toad's cousin makes a living selling maps in Desert Land.
In The Adventures of Super Mario Bros. 3 episode "Mind Your Mummy Mommy, Mario", Queen Mushroomkhamen mistakes Mario for her son and takes him back to her tomb, leaving Princess Toadstool, Luigi, and Toad behind. Not knowing how to find their way through the tombs of Desert Land's pyramids, Princess Toadstool comments that they need a map of the pyramids to find Mario. The mention of maps reminds Toad of his cousin, and he brings Princess Toadstool and Luigi to see his cousin. Wanting to make several sales, Toad's cousin advertises the maps to Princess Toadstool, including maps to the pyramids, mummy tombs, and even the houses of Mushroom celebrities. In the end, he sells only the map to the mummy tombs to Princess Toadstool.
Although Toad's cousin is never seen again, Luigi announces that he needs to find him again, so that he can buy a map to "someplace else" after Queen Mushroomkhamen has mistakened him for her husband.