Translation:This farmer has nine cows and eleven horses.
First of all it dieser is in Sg. Maskulinum. Dies- is a weak pronoun and takes the form according to gender and case. These pronouns have the same ending like definite articles (weak inflection). Another examples are: jen- (that), jed- (every), manch- (some) and many more.
If you need table and more discussion: https://language-easy.org/german/grammar/pronouns/demonstrative/ Discussion on duo: https://forum.duolingo.com/comment/34512894/Dies-Diese-Dieses-Dieser