Like Ereo said, pepper rexes don't drop the skeletal tail, only the tibia, ribs, and vertebra.
However, you can get the tail pretty easily, actually. When the wiki says "tilling in mines or Skull Cavern", it means the actual mines, not the mountain area. Go to a mine floor with some dirt on it (e.g. 5 or 11), and hoe all the dirt. If you don't get it, go up to floor 0 and then back to the floor you were on, to reset it, and keep hoeing. Rinse and repeat. Takes a fair amount of energy, but if you're at the point where you can go to Skull Caverns, you should be able to afford some salads from Gus. Just keep doing that till you get it.