Learn Finnish: Tämä koira tulee hulluksi yöllä.

This is the translative case! You add -ksi to a word when it describes a new state that someone or something is changing into. Because the dog is changing into a crazy state, hullu becomes hulluksi.

I thought tulee means "comes". Why does the English translation say "goes crazy"?

The verb tulla normally means "to come". However, when you pair tulla with a word in the translative case (like hulluksi), the meaning changes to "to become". So tulee hulluksi translates literally as "becomes crazy", which we naturally phrase as "goes crazy" in English.

Why doesn't the subject tämä koira take the translative ending too?

Tämä koira is the subject of the sentence (the one doing the action), so it stays in the basic dictionary form (the nominative case). The -ksi ending only goes on the word that describes the result of the change (hulluksi).

Why is "at night" translated as yöllä instead of using a separate word for "at"?

In Finnish, time expressions often use case endings instead of separate prepositions. The word for night is yö. Adding the adessive ending -llä (which often means "on" or "at") turns it into yöllä, meaning "at night" or "during the night".