My idea for Nether Elk is that when traveling through the nether forest, you see a flame that moves past your screen. You look and see a Elk with a charred coat, blue flaming eyes and antlers lit with blue flame. This would be the blue nether forest variant. The red nether forest variant would replace the blue eyes with yellow eyes and blue flame antlers with red flame.
The Nether Elk would be tameable using nether wart and/or nether berries that were teased in Minecon 2019. If the player were to attempt to tame the Nether Elk without the appropriate taming food, the Nether Elk would attack the player using a knockback attack that took away an initial 2 hearts and dealt fire damage over 3 seconds. The Nether Elk would be rideable without the use of a saddle or anything else, but you could put carpet and armor on it to customize and protect. The size of the Nether Elk would be larger than Horses, but nothing to massive. When riding a Nether Elk, Piglins will stay passive whether or not you are wearing golden armor. This also works for piglin beasts, but they will flee rather than stay passive.
The Nether Elk would be found wandering the nether forests mainly alone, but sometimes could be found in a herd. They would make noises such as raspy bleets as they have been charred due to long exposure to the Nether. They would run faster than most horses and faster than players.
Killing a Nether Elk wouldn't prove as to much of a benifit as it would only give you leather, meat and gunpowder.