Mystical World adds a ton of new creatures to Minecraft. It makes the world a bit more immersive and most of them can be tamed offering a greater variety of companionship and protection in the game. Foxes are typically aggressive to chickens and rabbits so keep watch of any you have at your bases. But they can be tamed and help you similarly to wolves.
You may also encounter deer and stags in forests. They drop venison and leather but otherwise are an adorable feature to wooded areas.
And silkworms are a new way to obtain string. When you break leaves you may get silkworm eggs that can be hatched and then over time they will form a silk cocoon that can be converted to silk thread.
You can also get ink from squid by right-clicking them with an empty bottle. This ink can be used as a substitute for black dye so you won’t need to kill any squids with this method.
The Nether also gets a new creature in the form of lava cats. You can tame them like ocelots and they have two forms: lava and obsidian. When they get wet they turn into their obsidian form and while they may be slower, their attacks are harder so it could play well in sticky situations.