I'm working on a command that will basically allow you to have a basic companion of, in theory, any mob. I just need to figure out how to make this dog ride an Iron Golem:

Wolf {CollarColor:0,Owner:<PLAYERNAME>,Sitting:0,Invulnerable:1,PersistenceRequired:1,Silent:1,ActiveEffects:[{Id:14,Amplifier:1,Duration:19999999999980,ShowParticles:0b}]}

Please send the command of this dog riding an iron golem, it will help a lot!