CapriUni - 'Meerhäschen'(= small 'sea rabbit' or 'sea hare') is unknown to me and so is the story you mentioned. If it is from the Brothers Grimm I will have to admit that I don't know their stories.
What I know is a 'Meerschweinchen' (= small 'sea pig'). I had to look up the english expression 'guinea pig' in a dictionary. At least english and german agree that the rodent in question should be thought of as some kind of pig...
If I'm not completely unaware of my native tongue I can state that 'Meerhäschen' is deliberate nonsense. A correct translation should be 'guinea rabbit' (the same way as 'Meerschweinchen' was perverted to 'Meerhäschen'). Why 'mongoose'?
Hope that helps.