The Brazen Horse of Troy and other Google hits are proving it I think.

maybe something like hot-blooded, fiery, spirited?

Shergar: 25 years on, the mystery that won't go away - Yorkshire Post
29 Jan 2008 ... However, he pointed out there was a world of difference between caring for a child's pony and a highly-strung racehorse. ...
www.yorkshirepost.co.uk/highlights/Shergar-25-years-on-the.... - 53k - Cached - Similar pages

Farmers Guardian - Retraining racehorses to become riding horses
18 May 2007 ... “Foals are looked after like gold dust and you’ve got to remember they’re very highly strung.” Racehorses are broken in differently to ...
www.farmersguardian.com/story.asp?sectioncode=51&storycode=... - 24k -

Newmarket Connections, Newmarket tours, Newmarket races pubs bars ...
Racing’s people face potential dangers whenever they are handling the highly strung Thoroughbred racehorse. The long hours in adverse weather conditions can ...
www.newmarketconnections.co.uk/ - 32k

Meet the Toyger, the must-have designer cat that's been bred to ...
15 Oct 2007 ... "As well as being beautiful they have a wonderful nature and are perfect for people who don't want a highly strung cat. ...
www.dailymail.co.uk/pages/live/articles/news/news.html?in_a... - Similar pages

Catteries: fab listed boarding catteries (2)
"The staff are very friendly, even with my very highly strung cat." "I have used other catteries in the past but this is by far the best my cat has ever ...

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Note added at 1 hr (2008-04-21 12:49:49 GMT)
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"alert" might work better

I fell in love with her sweet disposition and I learned that Arabians were not the high-strung, hard to manage breed that I had heard; but, instead, were extremely sensitive to human feelings, highly alert to their surroundings, and very lovable.
http://www.terrellarabians.com/

la definizione che hai trovato tu è secondo me un po' parziale, o meglio, descrive in un certo senso le "conseguenze psicosomatiche" del carattere nevrile.
Infatti, nevrile identifica sia il carattere sia il tipo di comportamento (e quindi aspetto e tipo di movimenti) che ne deriva del cavallo

per esempio
"Sensitivity
The next thing to assess is your horse's level of sensitivity. Many horses are very sensitive, but lack attention or awareness. A sensitive horse often over-reacts to all kinds of stimulus from sounds, smells, sight, taste and touch, as well as diet and other changes. He will often have tense muscles if he internalizes his stress reaction."

http://www.todayshorse.com/Articles/AssesingYourHorse.htm

e anche


http://www.google.it/search?num=30&hl=it&safe=off&q=sensitiv...

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Note added at 11 hrs (2008-04-21 21:58:56 GMT)
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ah, chiaramente solo 'sensitive' :)
e so che sembra molto lontano da 'nevrile', eppure è il termine più appropriato e preciso che ho trovato per renderne appieno il senso

I think that up tight, high strung are good choices but the closest word you are looking for would be nervy.