Isst is eat and for humans and frisst is feeds on and for animals. At least thats what I was told
Because Hund is masculine. Masculine and neuter use 'mein', whereas 'meine' is used for feminine words.
In English there is only one verb for eat, but in German there are two, one for people (essen), one for animals (fressen). "essen" and "fressen" simply mean eat, nothing else .
Der Hund frisst = the dog eats.
Der Mann isst = the man eats.
I did the voice thing where you say it, and I said it no different than the voice and it marked it wrong three times.