In life, Nathanos was a human who saved a high elf called Sylvanas Windrunner from an Amani ambush. When Sylvanas became the Ranger-General of Quel’thalas, she declared that Nathanos should be trained as a ranger. Nathanos was the only human ranger and he was talented enough that he even became a ranger lord, one of the Ranger-General’s lieutenants. However, he attracted a lot of controversy so rather than cause trouble for Sylvanas, Nathanos left for his homeland of Lordaeron.
Nathanos was killed and reanimated as one of the undead by the Scourge, during the Third War. Sylvanas, who has also been killed and raised as an undead dark ranger, broke free of the Scourge’s control and managed to liberate Nathanos from their control as well. He became the champion of Sylvanas Windrunner when she founded the undead nation of the Forsaken on the ruins of Lordaeron and eventually, he became her second-in-command.
Eventually Sylvanas turned against the Horde and Nathanos defected alongside her, only for him to be killed by the night elven priestess of the moon, Tyrande Whisperwind, who wanted to avenge the mass murder of night elves that Sylvanas had perpetrated. There’s more to the story but that’s the most relevant information at the moment.
Articles you may want to look at:
https://wowpedia.fandom.com/wiki/Nathanos_Blightcaller
https://wowpedia.fandom.com/wiki/Sylvanas_Windrunner
https://wowpedia.fandom.com/wiki/Forsaken
Nathanos has two aesthetics. One as a mouldering corpse who wears green and brown huntsman’s outfit made out of leather, one as a red-eyed vampire-looking fellow dressed in black and grey. In both cases, he’s known for wielding axes and a longbow and fighting alongside darkhounds, demonic-looking dogs that inhabit the ruins of Lordaeron.
After the Cataclysm expansion, he became a hunter trainer and is likely responsible for training a lot of undead hunters. The most famous undead hunters are dark rangers, who were almost all high elf rangers in life. However, there are some Forsaken rangers who were human when they were alive. They mostly sport a leather aesthetic and seem to focus on crossbows and axes as their weapons of choice.
https://wowpedia.fandom.com/wiki/Making_the_Rounds_(Horde)
Lady Sylvanas Windrunner says: Nathanos has been training a new generation of rangers. They represent both our past, and our future.
https://wowpedia.fandom.com/wiki/Forsaken_Ranger
So if you want to play a very traditional undead hunter, you could aim for a look similar to that and claim to be one of the many rangers trained by Nathanos.