Porque le dio entonces como un mareo...

[Quaeso]

Siguióle el buen fraile asombrado y quedóse al otro lado de la puerta, viendo por sus rendijas cómo el desván se iluminaba al abrir el chico, como de costumbre, las maderas del ventanillo. Pero no pudo ver más, porque le dio entonces como un mareo y a poco pierde el sentido y viene a dar con su gran cuerpo en el suelo. -Marcelino ch. 3

Is mareo the subject of dio, or is mareo the direct object?

[Quaeso]

In other words:

• "because something like a dizziness then struck him" -would be mareo as subject.
  
• "because it (the event of watching at the window) then gave to him something like a dizziness" -would be mareo as a direct object.

[poli]

It's D.O.

[aleCcowaN]

Le gustó la cena.

Le dio un mareo.

Exactly the same, neither the meal liked him nor the dizziness made itself feel in him.

Maybe this doubt was caused by the fact that dar has no direct translation in this kind of cases.

Me da miedo (I got scared -by something-)
Me da náuseas (it makes me nauseated, though a mass of folks insists in saying "It makes me nauseous")
Me da mareos = Me marea (it makes me dizzy)
Me dio un mareo (I experienced a sudden episode of dizziness)

[Rusty]

I'll add my .

When the verb 'dar' is used to express feelings, it functions similarly to how the verb 'gustar' works (grammatically speaking); i.e., the Spanish subject is the feeling, while the person experiencing the feeling is expressed with a Spanish indirect object pronoun. In the case of the boy who is experiencing dizziness, 'mareo' is the feeling (placed in the Spanish subject). The subject quite often trails the verb, but it may certainly precede it, with accommodation for the indirect object pronoun which must immediately precede the verb.

Did you note that there’s no mention of a Spanish direct object? That’s because there can never be a Spanish direct object when 'dar' is used to express feelings. That is precisely why Alec mentions that we English speakers have a lot of difficulty with these kinds of verbs.

He demonstrated that English speakers almost always have to choose another verb to express the Spanish verb 'dar' when it's used to express feelings. For example, «me da miedo» is often translated into English as "I'm scared" if there's no context; "It scares me," if there is (the pronoun 'it' standing in place of the context). In either case, the English structure must add/use words that aren't in the Spanish phrase.

--subject (I) - linking verb (am) - predicate adjective (scared)-- -or-
--established subject - some verb expressing the feeling - direct object

Either way, we can’t just say 'subject' without including the language and the grammatical function therein to be perfectly clear.

So, which word in «me da miedo» corresponds to the English predicate adjective or the verb of feeling? It’s the Spanish noun 'miedo' (fear). Why doesn't 'fear' appear in either English translation? A dilemma, to be sure.

Now, let’s look at the topic in the thread’s title.
Which word in «le dio … mareo» is the Spanish subject? It’s clearly 'mareo', the feeling.

Alec didn’t provide an English translation, so let’s use the one you proposed ("because (something like) a dizziness then struck him"). The feeling (a dizziness) functions as the English subject. Oh, and you referred back to the boy using the direct object pronoun 'him' after the alternative verb.
(Since the boy was the subject in the Spanish main clause, the Spanish adverbial clause's indirect object pronoun 'le' points back to the boy in the main clause.)

So, Alec was right in suggesting that the Spanish usage of 'dar', when used to express feelings, is difficult to translate "directly." There's nothing direct about it!

Hope that helps some.