"It is a dog."
Translation:Αυτός είναι ένας σκύλος.
why is "αυτός είναι σκύλος" not correct? Sentences like "είναι καθηγητής" (he is a professor) and "έγινε καθηγητής" (he became a professor) are ok!
There's this phrase, "(Αυτός/Αυτή) είναι σκύλος", but it's way too colloquial and may convey different meanings depending on the situation. It's often used in cases of hard-working people or people who manage to pull off demanding tasks. However, it really isn't the most elegant of phrases and, for that reason, it's better to leave it out of the list.
But, anyway, info aside, your suggestion means "He is a dog".