A big thanks to kireina_akuma for providing us with the information!
In trying to figure out what the purpose of the Symol Hole is (it never does anything for me), I started replacing numbers in the URL, and here’s what I came up with.
This is what a typical URL looks like for it: http://www.neopets.com/medieval/symolhole.phtml?type=end&e=0&rnd=986142
See where it says &e=0? That’s what tells you what happens.
** Please note that these are only descriptions. Placing them in your browser will not result in the prizes described below. The only way to gain the prizes is to actually play the game!
0: Nothing happens.
1, 2, 3: Gives you an item of some sort. What the item is I don’t know; since it’s not supposed to give me one (I believe the &rnd= number influences this) it wouldn’t show it. I’ve heard that someone got a window of sorts from it.
4: Raises your petpet a level.
5: Gives you a bit of NP; how much the most you can get is, I’m not sure. I’ve heard a number around 200 NP, so far.
6: As far as I know, nothing happens, but let me know if it does!
7 and above: Seems to have no effect (text doesn’t come up underneath “XXX jumps out of the hole a few seconds later).
Also note that you can now get the avatar for Symol Hole randomly when you send your petpet down the Symol hole.
Update: muchacha_tonto reports that once he received neopoints and once the Symol came up with broken pottery.