Leashed Silkworms are hand items introduced along with the Pet Battles update. It can be obtained by using a Silkworm Leash on an adult Silkworm with its respective colour (e.g. using one on a green Silkworm will give a Leashed Silkworm - Green). The Silkworm will disappear once the Silkworm Leash has been consumed.
When the Leashed Silkworm is equipped, it will summon a pet silkworm in its respective colour which follows the player around. If a player performs the /dance command, the silkworm will dance too.
There are 9 different possible colors: