In a very round-about way I found this interesting sculptor after watching of all things -a vintage film about the "new" (1968) Victoria railway in the UK.
As I was watching them detailed the new BlackHorse street station that was under construciton, and they showed a model of the station, then a horse profile sculpture plaque on the facade by the entrance.
Next it cut to showing the sculptor Dave Mcfall (1919-1988) modelling the original horse, and went on to detail that to some degree.

As the film progressed, it cut back to the horse twice more briefly, so I went to google street view and found the station, and the horse are still there.
Next I did alittle googling and found a web site on his work, he did a lot of interesting work, mostly figural and the horse was a little different from his other work.

I learned he developed a brain tumor, lost an eye and passed away just as he finished his last work- a bronze.
I found it interesting that he made the original horse from clay (Im assuming plasticene) then made a mold of it I htink, and cast it in plaster, and then the final was cast in fiberglass, not exactly my choice of a quality material...

I'm thinking he cast the plaster for at least one reason- to texture it as he is shown here with a textured hammer lightly working it over, tho this could be the clay too- hard to tell from the video exactly.




Another view shows him chipping away what appears to be the plaster mold to reveal the black fiberglass cast;




So he didn't make a rubber mold it seems, a bit odd to do this this way, maybe more definitive answers will come later.

Horse 184cm high x 216cm wide
The work was commissioned by the London Transport Board for the new Victoria Line underground. It was first modelled in clay, then plaster and finally cast in glass reinforced polyester.

Wow. You made me intrigued enough to google him too. The website dedicated to him and his art is very interesting indeed. A talented talented man. Thanks!!!!