Yes, he is. :) But, this way, you would be using "indefinite article" instead of "definite article" and the meaning would change a little.
A horse is an animal = Um cavalo é um animal
The horse = O cavalo
A horse = Um cavalo
The eagle = A águia
An eagle = Uma águia
Don't forget, "in Portuguese the articles can vary depending on gender and number."
In short that's it :)