"I have five dogs and five cats."
Translation:Έχω πέντε σκύλους και πέντε γάτες.
This is Plural, Accusative case :) (Οι) σκύλοι (nominative), (των) σκύλων (genitive), (τους) σκύλους (accusative).
If you're wondering why Accusative is needed here, it's because you're answering the question of "whom/what", as in "What do you have?". You've got five cats and five dogs.
It sounds a bit weird, but that's how accusative works. ^.^
Σκυλιά is a neutral plural that's used, just in case that the dogs' gender isn't specified, or if it's a group of dogs from both.
(Normally, σκύλος is the male version, σκυλίτσα -or σκύλα, although I don't recommend using that one. It's mostly used as a curse. :P- the feminine one, and σκυλί the neuter one, in cases where gender isn't specified or important. You can use whichever one you want, σκύλος/σκύλοι or σκυλί/σκυλιά, both are correct. ^.^)