Dylan is a child with an unknown gender.
When Marge is driving to Dylan's birthday party with Bart, Lisa and Maggie as passengers, she mentions that Bart used to play with Dylan as a small kid. However, neither Marge, nor Bart know who or what gender Dylan actually is. The child invites both, boys and girls, and seems to not like trucks.
Bart doesn't actually for this matter and soon starts to play a Japanese card game called Battle Balls with other boys at the party.