why the dictionary hint for "Seine" is: His (Feminine/Plural)? His is masculine!!! So, I understand that "Seine Katze" means he has a cat but Katze is feminine so i have to use the feminine form, although the owner is masculine...please, explain me because i am used to think in owner´s gender for demonstrative adjectives.
This may be confusing, but follow the links and you should hopefully understand it.
Sein is "his", but it has to agree with what it is you are talking about. Sein (his) is a possessive pronoun. In German, possessive pronouns are treated like indefinite articles (like ein). In this case you are talking about the "his cat" (die Katze). Katze is feminine. So "sein" has to agree with "Katze". Sein will have to have a feminine form (in grammar only...it is still "his" and he is still a man). The last thing to check is what case it is in. This is the nominative case, so if you check the table in this link, you'll see that under "feminine" and "nominative", you add an -e. So it is "Seine Katze".
Here is a link to cases if you need to read up on those:
Hope that helps!