In my opinion in mountain or extreme hills biomes to less animals, so I got the idea to add a new one: the chamois.
The chamois would be a neutral mob climbing around at cliffs (it should jump two blocks high and get less fall damage) and searching for berries or other types of plants. If the chamois finds any kind of plant it eats it and started cimbing again. If the player holds berries or an carrot in the hand the chamois starts to follow him. You can also try to tame it with a berry (20% chance). If you've tamed it you can ride it (that would make it much easier to travel in mountain biomes).
You can breed two chamoises with carrots. If you hit a chamois it would tries to kick you from the mountain (it wouldn't give you much damage, but a pretty hard knockback). If you kill a chamois you get leather and as a rare drop you can get one of the horns (that could be used for a new potion).
You can write in the comments below if you like or don't like the idea or any suggestions you can write there, too. If you have an idea what the potion with the horn could do, than write it in the comments. I would be so glad if a blocky chamois would jump around at the mountains in the future :-)