Actually there are rules about this, you can find more about it in the tips feature on the desktop version of Duolingo.
Here's a tip though.
Dog is 'cù'. Dog after the words 'aon' and 'dà' is leniated to become 'chù'. Even though two dogs is plural, it's still written as singular after the word 'dà', just leniated. I don't remember why they don't use the plural for dog after two, they just don't. More than two dogs, and it becomes 'coin', the plural word for dogs.
Hope that helps.