"It is a horse whose name I don't know."
Translation:Es ist ein Pferd, dessen Namen ich nicht kenne.
Because it's a subordinate clause, and that kicks the verb to the end. If it were a main clause, it'd be "Ich kenne (den) Namen nicht." But because we have the relative pronoun "dessen" (whose), "kenne" stays conjugated and goes to the end: ...dessen Namen ich nicht kenne.
I don't know of a resource for general rules on noun declensions.
When I am in doubt about the declension of a particular noun, though, I have a look at canoo: http://www.canoo.net/.
It also lists inflected forms of verbs (handy to check whether a given imperative can take both forms with and without -e, for example) and other inflectable parts of speech.
WHY Kennen and not Wissen?? I thought that you are supposed to use kennen when you know a person, someone and wissen for objects, things, names, etc. Here we are talking about knowing a name which is thing. Why use "kennen" instead of wissen?? Could a native speaker explain this to me? I would appreciate that very much.
What I wrote is wrong. I do not delete , because mizinamo answered to it. What I wrote was:. „I think it has to be "Name", as long as the horse has just one single name. "Namen" as Genitive Case is Plural. Ein Pferd, dessen (zahlreiche) Namen auf der Urkunde vermerkt waren, hat das Rennen gewonnen ...“
The volunteer course contributors have provided tips and notes for this unit -- you may be able to access them here: https://www.duolingo.com/skill/de/Relative-Pronouns/tips-and-notes
However, our tips and notes are being replaced by just "tips" written by someone else (I'm guessing Duo staff), and those tips are not available for all units yet -- only for the first dozen or so.
Hopefully more tips will be added eventually and they will be as useful as possible.
@tangavango: I can't explain the "why", but I can tell you, how certain words usually are used.
When I don't know a person's name I usually say:
Ich weiß nicht, wie er/sie heißt. This sentence cannot be misunderstood.
Ich kenne seinen Namen nicht has two possible meanings. It might be that he has a very rare name I never heard before. ( = I am not familiar with the name). But it can also mean that I just don't know his name.