You cannot tame a Foxhound in the Nether unless you first obtain one through breeding or spawning methods that work in your modpack/version
Find a Foxhound in the Overworld
Feed it coal until it is tamed
Use a lead to bring it to the Nether if your modpack allows it
Keep it safe from lava, fire, and hostile mobs
If your version requires it, use the same taming method in the Nether after transporting it
Check your modpack’s Foxhound behavior, since Nether rules can differ by version