The answer is wrong if it's a horse that appears, or a car or a plane He appears at night. to people It appears at night. to animals or a car
Really it is more complicated than that in English. Most definitely if the animal is a pet then the animal is referred to as he or she. There are more and more now referring to all animals as that when the gender is known (and sometimes when not).
For cars, boats/ships and a few other things (like countries and cities), they are often referred to as she (this despite women were not allowed on boats for centuries – at least as crew – with the idea it would be bad luck). Then of course there is Lady Luck, and Lady Justice, and Mother Nature.
simply a rule to be followed (like in the morning/afternoon/evening; at night)
Why not, He appears in the night?
When I put that, I was told the correct answer is, He appears during the night and I see no Portuguese equivalent for "during" in Ele aparece à noite. But all three, including He appears at night mean essentially the same thing, and none of them is an exact direct translation.
This sentence doesn't mean anything in English - it's nonsense. You could add a noun to make this work, i.e. "He appears on the night show", but that turns night into an adjective.
I meant it in the sense "He will show up on that night," e.g. "He may not make it to any rehearsals, but he appears on the night." Sort of. Actually, reading it back to myself now, it does sound a bit dumb... :'(