Giant snails could be a rare mob that spawns in the swamp and forest biomes. They move slowly and remain firmly affixed to the surface they are crawling on. They are not knocked back when attacked. They can climb vertically and upside-down on full blocks. They are attracted to leaves and sit “eating” leaves and tall grass. They can be bred with leaves. They drop slime which can be crafted into slime balls or used to brew potions of sticky. The sticky effect makes the affected entity stick to and slide down all blocks as if those blocks were honey blocks. Giant snails could also drop snail shells which could be used to repair turtle shell helmets or could be used instead of nautilus shells in the conduit crafting recipe.
A giant snail mob would be a good addition to Minecraft because it would teach kids about the real-world animal, it would add exciting life to these biomes, it would add another source of slime balls, and it would add a new potion and effect to Minecraft that would be useful in map-making.