Mob Idea - Turtle

And here's another reptile mob
I tried to balance this one compared to other passive mobs, I hope I did good enough.

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Turtle
Alignment: Passive
Health: 8 (4x )
Damage: N/A
Spawn: Swamp (within 50 blocks of water)
Drops: 0-1 Reptile Hide (Green), 1 Shell
Experience: 2-3
Turtles are amphibious armored reptiles that walk at the same speed as a player's sneaking movement on land, and swim about half as fast as a boat in the water. They are preyed on by Gators, which can crunch their shells like crackers, so they tend to move farther away from the water than their predators. They're also much more peaceful, preferring to hide in their shells when danger threatens - you can't damage them while they're hidden, so if you do plan to kill one, you'll have to wait for it to hack away at the shell with an axe (Axes are the only thing that can break turtle shells, but they do it very slowly). The shell actually goes through a breaking animation as you chip away, though you have to go through 10 stages of damage before the shell is broken completely and the turtle's death animation is played.
If you hold a majestic butter flower/dandelion/yellow flower, you can get a Turtle to follow you, and you can breed Turtles with dandelions as well (baby Turtles take twice as long to grow to adults as other passive mobs, though). Additionally, these Turtles are large enough that you can slap a saddle on one and ride it, and though it's obviously much slower than a minecart or boat, it also doesn't take damage when hitting things, making for a safer and more leisurely trip. You can only steer the Turtle if you're holding a butter flower, though.

Shell
If you break a Turtle's Shell, it drops as an item (though admit it, you'll probably feel sorry for the poor Turtle). Mechanics-wise, this Shell is essentially a dyeable iron helmet, and the color is dependent on the color of the turtle it comes from. (You can dye Turtle shells like Sheep wool while they're still on the turtle. That's Minecraft logic for you!)

Turtle Egg
A Turtle lays an Egg exactly once per Minecraft day. You can collect these eggs and use them as a substitute for regular chicken Eggs, including throwing them to hatch baby Turtles. However, only one in every 32 thrown Turtle Eggs spawns a baby Turtle. This, combined with the fact that Turtles grow very slowly, makes it much easier to just use butter flowers to breed them in the standard Minecraftian fashion.

Minecraft (c) Mojang