how can i make this mob spawn naturally?

summon zombie_horse ~ ~ ~ {SaddleItem:{id:saddle,Count:1},Tame:0,Temper:20,Health:30,Attributes:[{Name:"generic.follow_range",Base:64f},{Name:"generic.attack_damage",Base:15f},{Name:"generic.max_health",Base:30f}],Passengers:[{id:zombie,CanBreakDoors:1,InWaterTime:200,CustomName:'[{"text":"Zombified Monster Hunter","color":"#ff0000"}]',Health:100,Team:undead,active_effects:[{id:strength,duration:3,amplifier:99999999999,show_particles:0b}],HandItems:[{id:iron_sword,count:1},{id:shield,count:1}],ArmorItems:[{id:iron_boots,count:1},{id:iron_leggings,count:1},{id:iron_chestplate,count:1},{id:iron_helmet,count:1}],Attributes:[{Name:"zombie.spawn_reinforcements",Base:4f},{Name:"generic.knockback_resistance",Base:10f},{Name:"generic.attack_damage",Base:14f},{Name:"generic.max_health",Base:100f}],HandDropChances:[0f,0f],ArmorDropChances:[0f,0f,0f,0f]}]} want this mob to spawn rarely in the overworld