Potential treasure guardians

1. Griffins. They are well known for their ability to protect treasure, and you don't get a reputation  like that for sleeping on the job.
2. Dragons. These terrifying monsters actually can get away with sleeping on the job. Because really, would you risk waking a dragon just to get some gold?
3. Leprechauns. These guys usually specialize in gold, and some can even make the gold vanish if someone takes it. But they aren't really the best security-wise, since all you need to do is find the end of the rainbow.
4. Sphinx. A sphinx can be wonderful at guarding the way to your treasure. But be warned: if you can't solve their riddle, then they might just eat you. It doesn't matter to them that the treasure is yours.
5. A siren. If your treasure is hidden on an island, then a siren might be your best best at warding away ships. Just remember your earplugs.
6. Hydra. If one head is good for spotting intruders, then just imagine how much better a multi-headed monster could find them. And even better, pesky thieves can't simply behead it and be done, since it takes more than a sword to stop a hydra head regeneration.
7. A will-o-wisp. Just imagine some bad guy trying to find your treasure. Now just think what would happen if the corridor was lit with will-o-wisps, who led the bad guy into a trap. Pretty good job for something doubling as the lighting.
8. Unicorn. Such a pretty creature, thieves will think it harmless. But unicorns have a long, sharp horn, and deadly hooves. They also don't usually take well to hunters, so treasure hunters beware.
9. A phoenix. Just imagine a lovely trap just before the room where your treasure is stored. It is a room that bursts into flames should the trap not be properly disarmed. This lovely trap would of course be fueled by a phoenix's inner fire, and the disarming of said trap would involve pleasing the phoenix.
10. A cat. Because what could be more frustrating than a cat who ignores anyone trying to enter the vault. Especially when the cat is the only one who can open the vault.