Goldfish

Goldfish in Minecraft would spawn in most ocean biomes, probably all except for frozen oceans. They would be about as small as pufferfish, until you feed them. It would be a gradual process, and go through four or five size stages. In real life, goldfish get bigger the more you feed them. The in-game process would be gradual and there would probably be a cooldown for feeding so you can't rush and get a big goldfish, and you would have to form a bond with your goldfish. The benefit of having a big goldfish would be that you could put a saddle on its back and ride it underwater. To breed goldfish, it would probably be the usual way, feeding them, and they grow gradually too. Their breeding food could be a new underwater plant, or an existing one, like dried kelp. (because kelp is so easy to get, and you would have to put some effort into smelting it). Riding a goldfish would be more like flying an elytra than riding a horse, because you can go up and down in water. Maybe there could also be a new item that could help you control its speed while riding. Thank you!