ita, I'd say most modern breeds don't look much like they did in the 1800s, and your best bet's going to be to look for pictures of 1800s horses.
Horse
[link] (Okay, wrong era, but Impala = Bucephalus)
Calvary, basically. Look for pictures of calvary mounts.
The author has picked Friesian from the suggestions you guys gave yesterday, so I'll stick with that.
I am *so* tempted to take liberties with the tack. I mean, it's a car! Chrome grill very important here.
If the Impala is magically turning into a horse, I think the tack could be magical, too. Have fun with it.
If the impala turned into a horse and carriage there's all sorts of flashy bits people used to put on to ward off the 'evil eye'.
Check this out.
Although, really - could anyone do any better than Gericault? or Degas or Stubbs.
Chrome/big ass metal decorations at the corners of the bit, ita. Period-appropriate and you could TOTALLY sneak in devil's traps and stuff.
Arthur's a cariage horse but I think you could take some liberties with riding tack. Note the forehead ornament.
There would be nothing keeping your character from having a Spanish bridle from Mexico or California: [link] [link]
Yeah, lots of silver would totally work.
And the horse could have "high chrome" too - but that would have been frowned upon by many in the 19th century.