Learn French: Le chien est fort, mais le chat est petit. - The dog is strong, but the cat is small.

Because they are describing le chien (the dog) and le chat (the cat), which are both masculine nouns. You only add an -e for feminine adjective agreement. If we were describing a feminine noun, like la souris (the mouse), we would add the -e and say la souris est petite.

Do I pronounce the -t at the end of fort and petit?

No, in French, final consonants are usually silent, so you don't hear the -t in these masculine words. However, this is exactly why the feminine agreement rule is so important: when you add an -e to make the feminine forms forte and petite, that extra vowel forces you to pronounce the -t sound!

I know chien and chat are the default words for these animals. What if I want to talk about a female dog or cat?

If you specifically want to talk about a female animal, the nouns change to feminine: la chienne and la chatte. If you used those in this sentence, your adjectives would also need to change to match them, becoming forte and petite.

Is the -s at the end of mais pronounced?