In the nominative singular, hic, haec, and hoc refer to the masculine, feminine and neuter. They point to the object, in this case mustela, which must be feminine. Duo is not explaining Latin grammar to us.
I wrote: "Haec mustela est mea", which coude be translated as "This weasel is mine", and was marked wrong. I guess the word order do matters, after all. Even though "my" and "mine" are exactly the same word in Latin: mea