The Heirloom Legacy
When Nightmares and White Wyrms were introduced, they used the horse and dragon models and applied a hue overlay on them. This is why there was 4 Nightmare models and 2 White Wyrm models.
With Publish 11, released March 14th, 2001, they changed the way Nightmares and White Wyrms spawned. Newly spawning animals did not come in different models, but 1 single model to accommodate the Third Dawn 3d Client. . Tamers were upset with this change and in Publish 12, July 24th 2001 they reintroduced the different models of nightmares and white wyrms. However in doing so they changed the way they spawned. Instead of applying a hue to the base model of similar pets (horse/dragon), they created new based models for each creature. This meant that the hue overlay was no longer applied, but instead pre-hued sprites for the White Wyrm and Nightmare were created.
If you Hue Id a Nightmare or White Wyrm in the Enhanced Client you will see their base hue ID is 0, which means default color (for the plain colors, there are other hue ID's for the new colors released in Pub-97).
Nightmares and White Wyrms tamed before March of 2001 are what we refer to as Heirloom Legacies.
These pets will look different than normal in the Enhanced Client Only. Essentially they are double-hued and will appear more vibrant than normal.
Heirloom (Left), Normal Legacy (Right)
The same pets in the Classic Client:
From Left to Right: Heirloom Nightmare, Legacy Nightmare, New Nightmare (void/shiny), Dread Warhorse
In the Classic Client they look nearly identical, however you can see it in the trade window.
Normal Legacy Nightmare:
Heirloom White Wyrm:
Normal Legacy White Wyrm:
Looks aren't the only place where Heirloom White Wyrms differ from their currently spawning cousins. Their behavior differs also. White wyrms roar when commanded instead of grunting like all other white wyrms.
Pre-Publish 16 Legacies:
Publish 16 introduced a lot of changes to animal taming. The same patch that gave us armor itemization gave us visible pet stats, trainable skills, stable slots, pet bonding and control slots. Accounts that were active in this time period had their pets "normalized".
Nightmares that were tamed before Patch 16 and active at the time the patch hit had their Hits normalized to 315 and their strength normalized to 515.
One of the interesting things that happened were to accounts that were closed at the time Publish 16 hit. When those tamers returned, Their pets were normalized without physical resist and possible higher than normal other resists. Below is a nightmare that was tamed prior to Publish 16 but inactive when the publish hit servers.