I made a world with undead stuff like zombies and skeletons. I even got skeleton and zombie horses but I want to add a skeleton dog or wolf (whatever you want to call it) in it but I can't figure out how. thanks.
There are no skeleton dogs in vanilla (unmodded) Minecraft.
To get skeleton dogs alongside regular dogs, you will need a mod, that will add such a new mob (or variation).
If you don't need normal dogs, you may just use a resource pack that will replace normal dog's texture with skeleton. The dogs will still behave as usual, but look different.
I'm not able to recommend any specific mods or resource packs now, though.
If you want a pack that has skeleton wolves, you can try the HyperPack, which adds skeleton wolves and other nice add-ons (https://minecraft.curseforge.com/projects/hyperpackpvp)
If you want to get a skeleton dog/wolf in Minecraft, is a mod. I mean, there is absolutely NO WAY to get a skeleton dog/wolf in Minecraft vanilla (unmodded).