I myself have a Great Daen and I find it my duty to defend it's honor.
This breed was used for guard but not an ordinary on -for the German
Kings and other countries(China,for example)emperors.Great Danes are really proud and magestic but they are excellent athletes as well so they are very agile and muscolar and even quite attractive.
This is why that dog is thought to be and called as well the Apolon of all
dogs and also-the King.His mentality and spirit are very strong and unbreakable and in dangerous situations they don't doubt even for a second.They are also bent on their owners or friends (which is like the book)because if you let a dog be your friend(and especially a Great Dane)then he remains forever.This dog never leaves a person in danger.
Its physical and mental profile can be endless as well as its history and its role in it because he was the emperor dog.
I think this breed greatly coinsides
Tolkien's character in spirital and mental point as well.Moreover,Great Dane was used for a character in Sir Arthur Conan Doyle's novel 'The Houn of the Baskervilles' so it could be used and here as well,but I don't think so.
First of all,I don't think Tolkien meant
Huan's breed when he wrote that he was a wolfhound but probably he pictured his mentality and spirit.It's all known that hounds and
wolves as well have extremely strong spirit and will-this is how they survive so even if Tolkien had chosen this 'mixture' of breeds he probably has done it to describe us his qualities but not his specific breed.
Secondly,Tolkien has richly described Huan's character but not very well his appearance so maybe we shouldn't discuss how we imagine his body but spirit and which breed it belongs to.
And thirdly,we shouldn't eliminate the option that he can be a non breed dog or 'meles' as we call them in Bulgaria.That doesn't mean he is weaker,no even exactly the other way.Sometimes and even all he times those dogs are smarter and more manageable with life because they have to survive or just that they feel they are neglected because of their appearance makes them fight to survive among
men and pure breed dogs.
So to sum up,I do think that Huan can be a Great Dane,but it is more likely to think that he had no specific breed because I don't think Tolkien and many more people as well find it the most important quality for a dog to be an excellent one.