Interesting question. Your question raises some questions of context. Are you trying to make 'winter stallion' into one word, or are you trying to translate this as a phrase? Could the context in which it is being used have some bearing on the translation?
I do not know how much lingustic or second/multiple language background you have, but a general principle that many will quote you is 'translate the meaning and not the words'. Languages differ in how they convey meaning, even if they use the same basic building blocks (nouns, verbs, etc.) Insgive has made a very sound translation for you in terms of a standalone phrase, or even as a name. You might want to comment on context, etc., but I think Insgive has you on the right path.
In any case, m'athchomaroon! I hope you can contribute here on a regular basis. The Dothraki learners community is fairly small, and more learners are always welcome. We are always trying to build new tools and resources you can use to learn this fascinating language. Make sure you check out David's blog at www.dothraki.com where he introduces new words, and ideas, and encourages active participation in the learning process. A number of us also participate in a Dothraki IRC chat on Monday afternoons at 5 PM Pacific time (8 PM Eastern). An IRC web client is also available on this website.
Good point. I can see now that i might have explained a bit wrong.
In short i am trying to create one word from those two separate words.
The biggest issues for me is that grammar is not my strong side + english is not my native language...so those two combined tend to open up for a few misunderstandings ^^