1. On the hill stood an old tree.

2. On the hill it stood.
3. On the hill stood this.
4. On the hill this stood.
5. On the hill stood these.
6. On the hill these stood.

(Are #1 and #2 correct? What about #3 and #4? Which one is correct? What about #5 and #6?))

#1 is best.

The others are OK, but would not be used much. #s 3 and 5 (if used) would probably be best in poetry.