Why not 'ten mysz'?
Because "mysz" is femnine.
Rewind... how is mysz feminine?!
Well... exceptions. Yes, I know it doesn't look feminine, but it is. You just have to remember.
Mysza is feminine
"Mysza" isn't an official dictionary word.
I thought only masculines ended in consonants. What are the actual rules ?
Mostly masculine ones. But there are exceptions, like "mysz" here.
How do you know what the exceptions are? I am completely new to Polish.
Well, you could find some list, but on the basic level, you will just see them when you stumble upon them.
Why is mouse feminine; it ends in a consonant
As far as I understand it's an exception.