Test of player is riding an entity

testfor @a {RootVehicle:{Entity:{id:minecraft:pig}}}

testfor @a {RootVehicle:{Entity:{id:minecraft:pig,CustomName:"Piggy Poo"}}}

testfor @a {RootVehicle:{Entity:{id:minecraft:pig,Tags:["piggyback"]}}}

testfor @a {RootVehicle:{Entity:{id:minecraft:pig,CustomName:"Piggy Poo",Tags:["piggyback"]}}}

testfor @a {RootVehicle:{Entity:{CustomName:"Horsie"}}}

testfor @a {RootVehicle:{Entity:{Tags:["spit"]}}}