Can't give you any information about mutton as I'm not familiar with it (haven't had the opportunity to try it). But goat is very similar to venison from White-tailed deer. It si great when cooked medium rare and goes well with onion, and savory spices such as pepper, sage, thyme, oregano, rosemary, saffron, etc. Salt is essential, but not too much. Let the natural flavor shine through. Vinegar will remove or mask some of the gaminess if you don't like that flavor. Also, the flavor of the meat will depend in large part on what the goat has eaten. If it has eaten a lot of corn, then the flavor will be similar to beef. If a lot of wild plants are it's normal diet, then be prepared for a very strong flavored meat.
A good idea is to cut a small piece of meat and quickly fry it with just a bit of salt and pepper. Taste it and it will give you an idea of what you will be working with, flavorwise.
Good luck to you and I hope you have a great culinary adventure.
Seeeeeeya; Goodweed of the north
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