The method we used with beaver (old one, weight 20 kg) goes as follows: first meat submerged 24 hours in vinegar-water-salt solution. Then, next 24 hours in marinade which in our case was red wine and carlic. It makes old beaver meat to taste ordinary game meat.

I have eaten norwegian whale meat (they nicknamed it as fjord ox) too and IMO it tastes like something between meat and liver.