The only requirements I know of indicate the creature must be at least one size category larger and "have an anatomy that could serve as a mount."
By RAW, a Black Bear could theoretically ride on an Elephant then, yes?
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Providing that the mount can serve as a mount and the rider can be a rider (e.g. snakes might require some special tack to be either a mount or a rider) that is the only criteria.
An army of pixies mounted on halflings mounted on humans mounted on horses mounted on elephants mounted on dragons is a frightening thing to have bearing down on you.