Since we're doing Slavic languages, let's go with Serbian.

- dog = pas (we don't distinguish between "a dog" and "the dog")

- declensions (singular): pas, psa, psu, psa, psu, psom, psu

- declensions (plural): psi, pasa, psima, pse, psi, psima, psima

- but that's not all, the number can change the declension: 1 pas; 2, 3, 4 psa; 5+ pasa

It's my language and I love it, but the grammar is a nightmare. I think it might be one of the easiest languages to learn to read and write, but good luck learning what it all means ;)