"This is my cat."
Translation:Αυτή είναι η γάτα μου.
I don't know if you are interested in Greek anymore, but here some information about the word for cat in Greek:
(H) Γάτα can be both, masculine or feminine. The special name of the masculine γάτα is (ο) γάτος. The female cat is γάτα. If you don't know the sex of the cat you can say simply γάτα. The kitten is called (το) γατάκι, sometimes used for cats of all ages, to express your love to a cat.
Probably because you may have typed in "Αυτό είναι το γάτα μου", which is inccorect. Η γάτα is feminine as a noun, while το γατί is neuter. So, Duolingo matched your answer to the translation that it was the closest to. :P
Γάτα usually refers to female cats only. Γατί either refers to a small cat, or a cat of either female or male gender. ^.^