Drew this enemy card...

http://hallofbeorn.com/LotR/Details/Hobgoblin-TWoR

And the card from the top of my deck was a trap. Is it considered attached to him as a trap? Or I have to defeat him first, then I can play it later as a trap?

Usually the card effects of the guarded attachments (besides the guarded keyword itself or any rules about how to claim it) are not active as long as it's guarded so I would say that in this case the Hobgoblin is not affected by trap, it's simply guardind it and you have to defeat it, claim it and then play it as a trap.

I disagree. Guarded cards get attached to enemies.

The traps effects are active, when they are attached to an enemy.

There is nothing in the rules insert, that states that the guarded card would be (considered) out of play or otherwise inactive.

Back when I first played Foundations of Stone, I asked the developer if Forest Snare's text was active when a Nameless Thing attached one to itself. The answer was no, and that the Nameless Thing would still attack; so I assume that would go for Guarded cards as well.

That might have still been Nate Fre nch though. 😀

You can only speculate about designer's intent.

But RA W I don't see any way for the rules to work that way.

Well, at the time, Caleb had actually already taken over as lead developer I was behind on products (still am).

Yeah, it's weird that whatever Hobgoblin guards doesn't count as an attachment, even though it technically is by the wording, but I guess that's just how it is.

The Nameless Thing issue remains in the FAQ:

Quote

Q: If a card is attached to Nameless Thing (D 125) as part of the resolution of its Forced effect, is that card’s game text active?

A: No. Cards that are attached to Nameless Thing (or Elder Nameless Thing) as a result of triggering its Forced effect are considered attachments with no card title or game text. The only active part of the attached card is its cost.

So by analogy, I'd guess that the answer to the OP question is, the trap's text is not active (essentially what @Alonewolf87 said).

While I think, the card's text is not active while attached to a Hobgoblin due to its forced effect, it would be funny, if it was: Imagine the Hobgoblin trying to loot some equipment and accidentally springing the trap.