I actually remember seeing a silver steed in game 99 or 98 maybe even but I am thinking 99 cuz that is when they made Siege, an my being awestruck by my own very first sighting of one. *had same reaction to nightmares when I first saw those put in game, although I got fried at the very first sighting of the nightmare was put into the game*

with the then new lost lands.
I was on Chesapeake back then, when I saw the first silver steed, but if memory serves it was supposed to be a Siege Perilous only sort of thing..ie hero good evil system silver steeds were the GOOD dark wisps (that still kill me on Siege) were EVIL being creatures if I recall. They made the steeds go extinct..maybe cuz they bled thru to other shards or... and were not supposed to be on the other shards, or cuz they knew then there were gonna make nightmares and unicorns yet, dunno. The hue of the silver steed kinda matches some of the old *hero* armor ye see still on Siege now and then which also can not be made *nor worn by any but *heros* any longer.
Here is a Stratics link for you for the extinct silver steeds, created on Siege part of Hero system, but no updates & stats missing. I think any of us whom ever saw one in game EVER like myself still remember them with some awe they were special, though. Pity they did away with them *yet they did not do away with them nasty Siege only, darn dark wisps* !
http://uo.stratics.com/database/view.php?db_content=hunters&id=321