Translation:There is a gray wolf running in the cold forest.
Bjarne, your 'is' is definitely out of place for good english. It is part of the verb 'is running' so they only sound comfortable together.
How about "in the cold forest a grey wolf runs"?
If you are a Tolkien fan the form of the word flow in Finnish is very natural, and often rather beautiful.
Technically, 'is' is its own, independent verb, in the indicative mood, while 'running' is just a participle, a verbal adjective. They don't necessarily need to stay together.
Consider "In the cold forest, there is a gray wolf running", and while much less usual, "Running in the cold forest is a gray wolf" is also possible.
I think "In the cold forest, is a gray wolf running?" would work alright for a question, but for a statement, either 'is' should follow the subject, or for a more poetic option, the participle precede the location.
Or the simple aspect 'runs' is an option, as in your suggested sentence.