Ridable Squid with Armor: Diamond, Gold, and Iron and Turtles shell can be reinforced with different materials. New saddle for Squid and turtles don't need a saddle. Able to use ink attack for propulsion and to hide from underwater mobs. Turtles can be used as an extra tank of oxygen underwater. Useful for underwater exploration. Sorry didn't add much detail just an idea.
I'm not sure about the rideable squid and all because the player would easily outweigh the squid (theoretically based on mass and size) and the squid's propulsion of ink would probably be reduced by slightly over 50%.
I do however, since we're talking about squids, think that maybe they should be "tamed" in a sense, not so much that they can be controlled (like dogs and such) but just for iconing breeding and such. They would be fed fish and would have two stages of growth (like most passive mobs) which would be youngling and adult.
I do like your idea of having a controllable ride animal, but this case, I think dolphins would be a better candidate if that and that the armor would have some kind of horn coming from the head region of the armour. On top of that, since dolphins are mammals, you would be able to see how much air bubbles they have when the player dives under water whilst on the dolphin so the player has to be cautious not to drown the dolphin while exploring that way they don't end up drowning it while diving underwater.
OOh and maybe the dolphin can be recalled by a player using some kind of whistle.
Why is this so? Well, here's why:
If a player ventures underwater and leaves the dolphin still in water, the dolphin would idle and drown to death. So the dolphin would probably swim around whilst maintaining oxygen. The dolphin would swim in a general area where the player last left them and could be recalled by a player using a whistle.As long as the player is within an x radius of blocks from the dolphin and is near a water source, the dolphin will swim near to the player when recalled using the whistle.
A whistle will ensure the non-idle dolphin can be easily recalled and not wander off too far away that the player loses the dolphin forever.
A whistle can also be used to do a stunt when the player is riding the dolphin.
If the player is underwater, the dolphin will swim to the player rapidly and wait for the player to get back on. If the player fails to get back on with a certain amount of time, the dolphin will swim back up to "regen" its air bubbles.
The dolphin cannot be recalled if the dolphin has low amounts of air bubbles and is regening this value, otherwise, the player can spam the whistle and kill the dolphin while at it. Consider this to be sort of like a cooldown.
These are just some ideas as well, not expecting inclusion into Minecraft as a feature, but I really think this concept would be a little neat addon for a possible next future update.
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