I can almost guarantee you that there wasn’t another toy released in 1981 whose box carried the description “Contents: one mammal.” Very much like the dewback creature from Star Wars, the smelly and none-too-smart steeds of the Rebels in The Empire Strikes Back were designed to be “ridden” by the Kenner figures.
(A bridle and reins were included with the tauntaun, but are not seen in these photos.)
Just like the dewback, the tauntaun’s back contained a “trap door” into which a figure’s legs could be inserted; the positioning of the saddle gave the illusion that the figure’s legs were now astride the tauntaun.
After the tauntaun had been on the market for a while, Kenner reissued a new version with a rubber belly that could be “sliced” open so a figure could be placed inside. Let’s see someone market a figure like that today!