I'm going to take a guess and say that a mother rat probably over groomed a baby, and chewed it's legs off. You should cull, if that's the case. A one legged rat won't be able to hold itself up to go to the bathroom properly, and won't be able to move much, or eat with it's hands, or stand up to use a water bottle, or dig in bedding, or so basically anything a normal rat could. I would say a 3 legged rat would be fine, but no way would one with only 1 leg.
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