Using the command

/summon magma_cube ~ ~1 ~ {Silent:1b,Passengers:[{id:"minecraft:zombie",Silent:1b,Invulnerable:1b,IsBaby:1b,ArmorItems:[{},{},\{id:"minecraft:diamond_chestplate",count:1},{}],active_effects:[\{id:"minecraft:invisibility",amplifier:0,duration:200000,show_particles:0b,show_icon:0b,ambient:0b}]}],active_effects:[\{id:"minecraft:invisibility",amplifier:0,duration:200000,show_particles:0b,show_icon:0b,ambient:0b}]}

On a commandblock at night summon a baby zombie riding a magma cube (small for this example). Both baby zombie and magma cube individually chase for the player but while the first is riding the second they got no interest in the player and they just roam randomly.

Thank you