Use

/summon minecraft:blaze ~ ~ ~ {Passengers:[{id:"minecraft:pig"}]}

to get a Pig riding a Blaze. Interestingly, it followed me when I had carrots in creative mode, and ran around like a headless chicken when I smacked it, but when I switched to survival mode, the Blaze tried to shoot me.

What I expected:

The Blaze should have taken all its cues from the Pig and been passive.