The easiest thing is to take "killer horse" at face value and say "horlock" means "kill" but I think that is far too simplistic and does not explore WHY Pratchett uses this (made-up) word.

So, better to explore possible origins and meanings for the word so you can perhaps invent a similar one when translating. I read the rest of the incident with the horse to get a better understanding.

The first word that springs to mind when I see "horlock" is "horlicks" the nourishing drink, which used to be given to children at nighttime to help them sleep, although it was also seen as an energy booster for athletes, explorers, the aged and infirm!

https://en.wikipedia.org/wiki/Horlicks

Thus, you could say this is a horse full of energy and fast (which it quite obviously is as it takes 4 men to hold him) and there was a very fast race horse called just that Horlicks
https://en.wikipedia.org/wiki/Horlicks_(horse)

"Horlicks (1983 – 24 August 2011) was an outstanding Thoroughbred racemare from New Zealand...." She won a lot of races!


The family name Horlicks gives a clue as to where the grey mare got her name

https://www.houseofnames.com/horlock-family-crest

The Horlo(i)ck surname comes from the Old English words "har," meaning "grey" and "locc," which refers to a lock of hair. Thus the surname was most likely formed from a nickname for someone with a patch of grey hair.

And the word "horlicks" is also used as a substitute for the profanity "bollocks".
https://en.wikipedia.org/wiki/Horlicks
"This substitution in the form of a singular noun is also used to refer to a minor disaster or shambles, as in "to make a complete horlicks of something". For example in July 2003 British Foreign Secretary Jack Straw described irregularities in the preparation and provenance of a dossier regarding weapons of mass destruction in Iraq as "a complete horlicks"...

So, what does "horlock" mean here?
It can mean trample, or kill, or bolt (run away fast)
(Moist) "Will he run?’
Hobson) ‘Not so much run as bolt, sir...

"A mysterious stranger Hobson had tried Boris as a racehorse and he would have been a very good one were it not for his unbreakable habit, at the off, of attacking the horse next to him and jumping the railings at the first bend.”


or "bollocks" meaning (to put it politely) injure someone's manhood (genitals) as Hobson asks Moist ‘Had all the kids you want, have yer?’ when Moist asks for the saddle to be taken off so he can ride bareback.

When Moist is holding on as the horse bolts away it says "Carefully, he eased more of the blanket under him. Various organs were going numb..."

It's a fast horse, doesn't want riders (though Moist manges to hang on)
"Boris...wasn’t too bad a ride. He’d hit his rhythm, a natural single-footed gait, and his burning eyes were focused on the blueness. His hatred of everything was for the moment subsumed in the sheer joy of space. ..at least he was headed in the right direction, which was away from his stable. Boris didn’t want to spend the days kicking the bricks out of his wall while waiting to throw the next bumptious idiot. He wanted to bite the horizon. He wanted to run. .."

Hope all this helps you come up with an idea for transltion



--------------------------------------------------
Note added at 3 hrs (2019-03-06 14:03:30 GMT)
--------------------------------------------------

OOPS sorry! I meant to close off the BOLD


--------------------------------------------------
Note added at 5 days (2019-03-11 10:54:41 GMT) Post-grading
--------------------------------------------------



Glad to have helped