Local directories?
"White's Directory of Bradford, Halifax, Wakefield, .... and other towns ... being engaged in worsted manufacture ... followed by general trades directory of districts" (1887 15th edition)
Has an index of places. Horton (Great and Little) are under the post town of Bradford.
"Post Office Bradford Directory" 1891 (other years available)
"Kelly's directory of the West Riding of Yorkshire" 1881 (There were various Kelly's Directories for Yorkshire and other editions)
"Directory of Leeds, Bradford" 1854 (I don't know if there were later editions)
"Ibbetson's Directory of Bradford" 1850 (ditto )
"Lund's Bradford Directory" 1856 (ditto)
Several directories with title "Leeds and Manufacturing Districts". Need to look at title page of each to see area included.
Slater's Directories
All above are in Library of University of Leicester Special Collections: Historical Directories of England and Wales. Pages can be viewed.
https://www2.le.ac.uk/library/find/specialcollectionsSome may also be in other collections. Some are on pay-to-view sites.
Added. White's Directory includes a street directory for Bradford with business proprietors and tradespersons in each street followed by an alphabetical directory by name.
You could make a list of streets in the neighbourhood of your ancestor's home and begin your search for mills that way. They probably lived within walking distance of work. There might have been a mill in every street.