Tameable Spiders

Inspired by the real-world symbiotic relationship where tarantulas will keep microhylid frogs to protect their eggs, Frogs should make all nearby Spiders completely passive, in which they'll only attack if a mob has harmed the Frog. While passive, wild Spiders can be tamed by feeding them Slimeballs, and can equipped with a Saddle. When riding them, players can climb blocks way faster than they could with a ladder—closer to the speed when swimming up a stream of water, making escaping deep caves and caverns way easier. Additionally, they'll attack mobs that have harmed the player, even while actively mounted. And they could also grant night vision when riding them in caves. The tradeoffs would be that they can't swim, have significantly slower walking speeds than horses, and are highly vulnerable since they can't be equipped with armour. Cave Spiders can remain the perpetually hostile variant, while the default Spider would basically become devoted cave-exploration mounts, like how Horses are for the overworld.