Lead

Leads are items used to hitch animals to fence posts or nether brick fence posts, and also to lead them around.

Usage and behavior
Right-clicking on an animal with a lead in your hand will tie it to the animal, allowing you to lead the animal; right-clicking on the animal again will remove its leash. You can use other inventory items while holding the lead, and you can have multiple leashed animals at once.

Once you have a leashed animal, right-clicking on a fence will attach any leashes you are holding to the fence with a visible knot. Attacking the knot or removing the fence post removes the leash.

A lead will stretch a maximum of ten meters before breaking. Whenever a leash is removed or broken it will drop as an item at the location of the animal; it will not automatically return to your inventory.

Most passive mobs can be "leashed". These include cats, pigs, chickens, cows, mooshrooms, horses, sheep, squids, Iron Golems, Snow Golems, and wolves. The Bat is the only passive mob which cannot be leashed. Note that leashed mobs that naturally despawn (like squids) still despawn.

Note that hostile mobs can in fact be put on a leash by modding the region save files. However, certain other mobs such as bats and players cannot be leashed, even in this way.

Trivia

 * The lead can extend out to 5 blocks when tied between a fence post and a mob.
 * They are named as 'leash' in the texture file.
 * Untamed ocelots can be captured and tied up using a lead.
 * If you are in creative mode you can leash a mob and pull it into the air, or tie it to a high up fence post (see picture below).
 * Multiple mobs can be tied to the same fence post.
 * You can use the lead to capture several mobs at the same time and walk them around altogether.
 * There is no known way of crafting them as of yet.

Gallery
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