SorryIn the sentence "In practice, no response at thisthe horse knew, twenty-four hours was usually about its lot."; "about its lot" is most likely referring to the horse and the 24 hours the horse has remaining; where remaining may be life expectancy, subject of interest, or the time horse is of use in some unstated context.
Since the horse is not introduced until the final sentence in the presented paragraph there are probably some other sentences which introduce the horse.
All in all this seems to be an example of poet usage.
The introduction statement "In Douglas Adams' Dirk Gently's Holistic Detective Agency, Chapter 2, when talking about how long the Electric Monk believed silly things, the book says:"; hints that the horse may be "how long the...Monk believed in silly things.
Hope this is helpful.
"..." means for brevity some word or words have been excluded.
This may contain facts, logic and reasoning that may be wrong.