"The dog is an animal."
Translation:Ο σκύλος είναι ένα ζώο.
I would say that there's no difference between σκύλος and σκυλί. They both translate to dog. If I had to mention a difference in usage, I'd say that some people use σκύλος exclusively for male dogs, while σκυλί could refer to a female dog as well (since the female σκύλα is also used a bit abusively, as a swear word, so people use this one less often when they are refering tot their pets.). But that clearly depends on the person. ^.^