A Plally (pronounced plal-eye) is a utility mob which can be grown in a farm. It's name is derived from "plant" and "ally".
Plally seeds can be crafted from combining melon seeds, wheat seeds, and pumpkin seeds in a crafting table. They can then be crafted, and will grow slowly. Once fully developed, they will grow into a two block-tall melon covered in vines. When the melon is broken, a Plally will spawn in it's place.
Plallies will attack hostile mobs and Endermen, much like Iron and Snow Golems. They will also defend against creepers by wrapping themselves around the creeper and letting it explode. This kills the Plally in the process but also preserves it's master's home.
Plallies will attack anyone who kills their planter.
Plallies have 15 health (7.5 hearts) and do 4 damage (2 hearts) with each attack. They are not capable of trampling farmland.
Combining the Plally seeds and an enderpearl creates a Plally capturer, which can be used on any Plally a player owns to capture it. The capturer can then be thrown, and the Plally will spawn and attack whatever the capturer hit, allowing it to be more versatile: attacking hostile players which have not yet successfully killed their master and being used to help kill passive mobs.