Gold Skeletons take on the appearance of a human skeletal system plated in an exotic gold-like substance. Gold Skeletons are generally similar to their white Skeleton counterparts, but they may have different statistics and drop items. In Castlevania: Portrait of Ruin it is mentioned directly that the substance they are covered in is not gold but an unknown exotic material, while in Curse of Darkness it's stated they're made from gold.
The first Gold Skeletons appeared in the Castle Treasury stage of Super Castlevania IV. Gold versions of the normal Skeletons, Bone Chukkers and Whip Skeletons appeared, although the only difference was their color.
In Curse of Darkness, Gold Skeletons (called Golden Bones) are just like regular Skeletons, but stronger and with a lot more of HP. They are the rarest enemies, appearing only in Baljhet Mountains (found rarely among a group of Skeletons near the last Save Room before the boss), Garibaldi Temple (found on the Inner Catacombs on the first hallway with Skeletons) and the Tower of Evermore (where he appears as the final enemy). The chance of finding Golden Bones is modified by the player's LCK, so the higher the attribute, the higher is the chance of meeting him. The hit points of Golden Bones surpasses the number of hit points of every other enemy in the game, including Nuculais, being 12,000. They can also drop (which is not 100% drop rate) one of the rarest materials in the game, the Miracle Egg.
In Portrait of Ruin, as well as in Super Castlevania IV, the Gold Skeleton's main method of attack is by wandering back and forth and lobbing gold plated bones at the player. The difference in Portrait of Ruin would be its increased mobility, unusually high defense, the number and strength of the bones it throws, and the ability to hop as most skeletal enemies do. These sheer nuisances are frustrating by themselves, but when grouped together, the odds of survival drops immensely.
Golden Bones is the rarest enemy in the entire game. They are only encountered twice. The first is among a group of Skeletons in Baljhet Mountains, before the boss. However, this is extremely rare, but if the player is lucky enough, they may encounter him in their first visit to the mountains. This might be the meaning of its bestiary description. It has 12,000 HP, more than any other enemy.
The second time is on the final floor of the Tower of Evermore. After he is killed, he will not reappear in the tower again. He acts like any normal Skeleton, and alone, he is actually easy to defeat, despite his high health.
In this game, the Gold Skeletons (not actually plated in gold but with another mysterious material) act similarly to normal Skeletons. They only appear in the Nest of Evil. However, these, with their powerful plating, are very resistant to attacks and have increased mobility, being able to hop around. Also, when they throw bones, they throw several at a time, and to make matters worse, they almost always are encountered in groups. Their attacks deal heavy damage, so caution must be taken when dealing with them. It's thus recommended to take them one at a time, and use weapons with Holy attributes, like the Holy Claymore or the Vampire Killer. The wide arcing pattern of the Holy Claymore (or any other two-handed sword, that is) can destroy most or all bones thrown by the Gold Skeletons in midair, and the long reach and double-hit capability of the Vampire Killer will allow to attack them at a safe distance. The Undead Killer is also an excellent option.