"His cats are pretty."
Translation:Seine Katzen sind schön.
Why "Seine" is correct, but "Ihre" is not?
Because the English sentence is about "his cats", not "her cats".
In another question "Her dog is dirty" it was "Ihr Hund ist schmutzig"
Exactly. That question was about something belong to her, not to him.
Given your name, I'm guessing you might be Ukrainian and that you might understand Russian?
"his cats" = seine Katzen = его кошки
"her dog" = ihr Hund = ей собака
his/sein/его are not the same as her/ihr/ей.
So, if it is Plural or Feminine it does get E-ending. but if it is Neuter( Whether it is singular neuter or plural neuter) it doesn't get E-ending. am I missing something?
That's not true, all nouns in the plural, including neuter nouns, add an -e. In fact, there is no grammatical gender distinction in the plural.
To summarize, "sein" is used with masculine and neuter singular nouns, and "seine" is used with feminine singular and all plural nouns.