Long ago, the Good/Evil patch was put in, though it is my understanding that it was truly never implemented on production shards (however, it was put on Siege Perilous, a shard created several months after the good/evil system was created). The story goes that for 2 or 3 days, this patch caused 4 new creatures to spawn that were never meant to spawn. They were the silver steed, dark steed, silver wolf, and dark wolf. Tamers were delighted and happily tamed them. When OSI realized the mistake, they did whatever they could to remove every single one of them. Stablemasters were coded to delete any one of those creatures upon stabling. Gamemasters (still) delete them on sight.
The silver and dark wolves had the same strength and stats as dire wolves. It is interesting in the fact that silver wolves would attack anything aggressive in the area. Killing a silver wolf might raise your fame, but it lowered your karma. Killing a dark wolf was the same as killing a dire wolf. While dark wolves had the same coloration as dires, silver wolves had a unique luminous color, the same color as silver steeds.
The silver and dark steeds had the same strength and stats as the current nightmares do. Long ago, nightmares were much more powerful and could kill dragons. When they became reduced to the strength of the steeds is unknown. Dark steeds were often mistaken for nightmares. Killing a silver steed will also result in karma loss in addition to gaining fame. Silver steeds only appear in the game today because their owners were clever enough to log out on them and keep them hidden from GM's eyes.
I had heard many stories about the steeds from Rhiannon and Xephyr of Napa Valley. They say that the steeds spawned in the same old overland regions as nightmares did, which means that they suffered from the same spawn mechanics. According to another tamer, usually the silver and dark steeds would spawn together and fight each other, as the silver steeds fought anything aggressive, just like the silver wolves.
With the extinction of silver/dark wolves and the near extinction of the steeds, stablemasters no longer delete them upon stabling, according to Xavier of Chesapeake. Apparently, he has stabled them many times and claimed them a few days later. It seems as though GM's no longer chase them down to delete. Calandryll commented on the legality of owning such an "illegal" creature, stating that as long as the owner of the steed does not put up a fight, all that will happen is the removal of the steed.
I have heard little of the original silver serpents. They are similar to the large ones you find roaming around swampy areas of T2A, only with the snake animation. According to the Stratics hunter's guide, they had natural deadly poisoning capabilities. Many people guessed that these creatures vanished from the game when the larger version of the silver serpent was introduced.