The easiest way to catch a nephelym is to just have sex with it, but there is another option using fluids that you can buy from venders or harvest from the nephelym that you have caught. I have tested all the options and each sub-race has one preference.
Bovaur
*Ayrshire – Vulwarg Milk
*Bull – Dragon Milk
*Minotaur – Vulwarg Milk
Demon
*Cambion – Seraphim Semen
*Incubus – Seraphim Milk
*Succubus – Seraphim Semen
Dragon
*Drake – Titan Milk
*Wyvern – Harpy Semen
Formurian
*Shark – Dragon Semen
Foxen
*Sionnach – Bovaur Milk
*Vulpuss – Vulwarg Semen
Harpy
*Bat – Dragon Milk
*Kestrel – Foxen Semen
Neko
*Tabby – Bovaur Milk
Risu
*Bunny – Thriae Semen
Seraphim
*Malakhim – Deamon Semen
Starfallen
*Carrier – Starfallen Milk
*Seeder – Starfallen Milk
*Spore – Starfallen Milk
Sylvan
*Elf – Seraphim Semen
*Goblin – Titan Semen
*Orc – Vulwarg Semen
*Slime – Hybrid Semen
Thriae
*Bee – Foxen Semen
Titan
*Colossus – Bovaur Milk
Vulwarg
*Lykos – Titan Semen
*Wulf – Foxen Milk
By Crowbar67