The two games that my brother and I have been writing about lately, Dragon Quest 8 and Persona 3 FES, share one thing in common: they both have really rare enemies that drop large rewards if you manage to kill them. In Dragon Quest 8, these creatures are the Metal Slimes (and variants) that drop very large amounts of experience points if you can defeat them. In Persona 3 they are the "gold Shadow" Wealth Hands (and variants) that drop rare items. Both are very rewarding if you can kill them, but the process can be very troublesome.
Both the Metal Slimes and the Wealth Hands share a number of characteristics. For one, they don't show up very often under normal circumstances. When they do show up, they can be really hard to kill. Metal Slimes are very hard to hurt and will run away at the drop of a hat. Wealth Hands tend to run away quickly before the battle even begins, and always get the first action (which they may use to run away). So, in the case that you find one of these rare creatures, one of the most common results is that they run away before you even get a glimpse of them, which is annoying to say the least. As such, these creatures can be hard to kill, but at least the rewards for doing so are in line with the difficulty.
Despite the similarities, though, I would say that I think Wealth Hands are the much better implementation of the concept, for a number of reasons. The most important of these reasons is that defeating Wealth hands is far less random than being able to defeat Metal Slimes. The greatest barrier to defeating a Wealth Hand is the way it runs away and vanishes before the start of the battle itself, and this is something that can be overcome if the player is cautious and uses a good approach strategy. What is more, finding them is easier because their gold coloration makes them stand out compared to other enemies, so you can reasonably process through a hundred enemies to find a Wealth Hand without fighting a single battle. To actively find a Metal Slime, on the other hand, you may need to tediously fight through a hundred battles. Finally, in the uncommon situation where an entire floor of Tartarus is full of Wealth Hands, the game turns into a Wealth Hand hunt, and defeating Wealth Hands can actually turn into a fun diversion from the normal routine of exploring Tartarus.
Another difference between the two kinds of rare monster is related to the rewards you receive for defeating them. The reward for killing Metal Slimes is a large amount of experience, which is certainly a significant reward in a game where levels can be hard to build up, but not a reward I would consider to be good. Having rare creatures drop very large amounts of experience makes the rate of gaining experience and levels far too random. Whether the player is behind the expected level curve, at the curve, or ahead of the curve can depend far too much on the random chance of a Metal Slime appearing and sticking around long enough to be defeated, which can be problematic. Even more importantly, experience points are not a very satisfying reward. They are important, but in my opinion they are simply too nebulous and vague to serve as a proper reward. Gaining levels is something that just happens across the game, so gaining more levels never feels very special. Wealth Hands, however, have a much better reward, since they are guaranteed to drop rare items. Whether it is a Medal item you need to complete a request, a Coin that you can sell for a lot of money, or a Nihil item you can use for Weapon Fusion, you usually get a great reward for defeating a Wealth Hand.
These kinds of monsters are a form of a rare opportunity for the player to get a rare reward. This is a good concept, but it is important for the people designing such a challenge to remember that they should not steal away a once in a blue moon opportunity before the player has enough time to seize the chance, and that the reward for such an effort should be worth the trouble.