Use
/summon minecraft:panda ~ ~ ~ {MainGene:"lazy",HiddenGene:"lazy",Passengers:[{id:"minecraft:piglin",HandItems:[{id:"minecraft:golden_sword",Count:1}]}]}
to summon a Piglin riding on the lazy Panda. But immidately Piglin get off.
Linked issues
Comments 1
ZYX_2D
Piglins not riding anything was considered to be intentional; see MC-172255.