Why is it that you make stock (and/or gravy) from a roast chicken or beef or fish, but you don't do the same for pork?
The flavor I think is why you usually don't see many other stocks.
Beef makes excellent general use stock, but fatty meat such as lamb, pork and oily poultry like duck and goose have too distinctive a flavour.
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I can attest to the lamb stock. I made a sauce from a lamb leg bone and the flavor was dominated by strong lamb taste.