Fibtraction - Fibonacci but with subtraction
Oasis, 4 bytes
c-21

Try it online!
0-indexed.
I saw this language used for a similar challenge once, and immediately I knew I should try it here. Can't say I'm disappointed.
Explanation
   1    a(0) = 1
  2     a(1) = 2
c       a(n) = a(n - 2)
 -                      - a(n - 1)