The horse's tantivy was fast as thunder

[BenDover23]

The horse ran tantivy as a thunder

Are these sentences correct? Is this word used often in the US and the UK?

http://www.yourdictionary.com/tantivy

[Haihao]

I suspect it's used neither in US nor UK very often because of the word tantivy. It is likely to appear in some novel.

[Beeesneees]

The term 'tantivy' to describe a horse at full gallop is pretty much obsolete now. It is only heard as a hunting cry (imitating what is being played on a hunting horn).
The cry is most often heard thse days as part of a traditional hunting song:
A-hunting we will go,
A-hunting we will go,
We'll catch a fox and put him in a box and never let him go.
Tantivy, Tantivy, Tantivy,
A-hunting we will go!

Subscribers at 'Wordsmith.com' confirm my views:
From: Francis Barnett (fbarnettATkamloopslawyers.com)
Subject: Re: A.Word.A.Day--tantivy
I've always thought that this word is imitative of the sound of the horn played at the beginning of an English hunt. The dictionaries do not seem to mention this, but there is a song which includes the line: "Tantivy, Tantivy, Tantivy, a-hunting we will go."
From: John Stifler (jstiflerATecons.umass.edu)
Subject: Re: A.Word.A.Day--tantivy
About "tantivy": If I remember correctly, the word was coined by British hunters as a vocal equivalent to the three distinctive hunting-horn notes played to rally the riders. On a piano you can get the effect by playing the notes C-F-A quickly three times in that pattern. Wanting to sing those same notes as if the horn call were a song, someone came up with "Tan-TI-vee, Tan-TI-vee, Tan-TI-vee."

Even if it were not obsolete, the sentence as written would not be grammatically correct with the phrase 'as a thunder' at the end.
Here are some example sentences which use the word in its traditional sense though:
http://www.wordnik.com/words/tantivy/examples
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Cheers m' dears!

[Beeesneees]

Before anyone gets confused by the use of 'a-hunting' in my previous post... this usage is also pretty obsolete now!
a- + present participle is an archaic English structure which is now usually only heard in traditional songs, nursery rhymes and poems. Here are more examples from ballads and rhymes:

A-roving, a-roving, a-roving is my ruin! I'll go no more a-roving with you fair maid.

She looked so neat and nimble - oh, a-hanging out the linen - oh! Dashing away with a smoothing iron, Dashing away with a smoothing iron, Dashing away with a smoothing iron, She stole my heart away.

"Where are you going to, my pretty maid?" "I'm going a-milking, Sir," she said. "May I come with you, my pretty maid?" "Certainly,fine Sir," she said.

Cry Baby Bunting, Daddy's gone a -hunting to catch a little rabbit skin, to put poor Baby Bunting in.
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Cheers m' dears!

[Haihao]

But I like this, BN. It gives a good explanation for the "go X-ing" form:

Go fishing << go a fishing << go on a fishing trip.