Star vs. the Forces of Evil: Mud Scene
Season 3, Episode 27, The Bogbeast of Boggabah
Star Butterfly was really getting impatient to find the Bogbeast and runs off to find it till she accidentally ran into the pit of mud she sinks. But later her father found her till she finds out the Bogbeast is isn't real only till she sank towards to her neck till got out
I hope I don't copyright by Disney.