Learn Russian: Собака — верный друг.

In Russian, the present tense of the verb to be (am, is, are) is almost always dropped. When you connect two nouns to say that one thing is another, you use a dash in writing to replace the missing word for 'is'.

Собака is a feminine noun, so why is the adjective верный in the masculine form?

Adjectives always match the noun they directly describe. Here, верный (faithful) describes друг (friend), which is a masculine noun. It does not matter that the subject of the sentence, собака, is feminine!

How do we know it translates to 'A dog' rather than 'The dog'?

Russian does not use articles like 'a', 'an', or 'the'. You have to figure it out from the context. Because this sentence is stating a general fact about dogs as a species, 'A dog' is the most natural way to say it in English.

Since собака is grammatically feminine, does it only mean a female dog?

No, собака is the standard, everyday word for a dog of any sex. Even though the word itself belongs to the feminine gender group, you use it to talk about male dogs too.