A showcase of 13 tricks in Minecraft that allow you to summon secret/hidden entities and mobs. Those are possible without the use of mods, you only need to have cheats enabled for the commands. Some of them are even possible in survival Minecraft! - https://www.youtube.com/watch?v=iQ8-QZSWWSY

1. Lightning Bolt - /summon LightningBolt
2. Giant - /summon Giant
3. Undead Horses - Zombie /summon EntityHorse ~ ~ ~ {Type:3} & Skeleton /summon EntityHorse ~ ~ ~ {Type:4}
4. Rabbit named "Toast" - /summon Rabbit ~ ~ ~ {CustomName:Toast}
5. Dinnerbone / Grumm mobs /summon Pig ~ ~ ~ {CustomName:Dinnerbone} - /summon Cow ~ ~ ~ {CustomName:Grumm}
6. jeb_ sheep - /summon Sheep ~ ~ ~ {CustomName:jeb_}
7. MinecartSpawner & MinecartCommandBlock - /summon MinecartSpawner & /summon MinecartCommandBlock
8. The Killer Rabbit - /summon Rabbit ~ ~ ~ {RabbitType:99}
9. Giant Wither - /summon WitherBoss ~ ~ ~ {Invul:25000,CustomName:Grumm}
10. Giant Slime & Magmacube - /summon Slime ~ ~ ~ {Size:20} & /summon LavaSlime ~ ~ ~ {Size:20}
11. Green robe villager - /summon Villager ~ ~ ~ {Proffesion:5}
12. Broken Horse - /summon EntityHorse ~ ~ ~ {Age:-50000}
13. _______

You might also like my other blog, 10 Secret Blocks ;)