Found a few more
Kick over the traces - A horse is harnessed to a carriage, wagon or cart with traces, which are the leather straps that run horizontally along the horse's sides. If through overexcitement or excess of energy the horse starts to buck and kick out, if it isn't quickly brought under control it may manage to kick right over the traces
Burr under your saddle
You can lead a horse to water, but you can't make it drink
Five dollar horse with a fifty dollar saddle
A camel is a horse designed by committee
Ginger up - to put ginger up a horse's fundament, to make him lively and carry his tail well
Hold your horses
A nod's as good as a wink to a blind horse
Don't change horses midstream
Don't lock the stable door after the horse has bolted
Pee like a race horse
Fit as a horse
Rode hard and put away wet
Happy trails to you
Shoo-in: a horse expected to easily win a race
Put through her paces
The big apple - "The Big Apple. The dream of every lad that ever threw a leg over a thoroughbred and the goal of all horsemen. Thereâ€™s only one Big Apple. Thatâ€™s New York."
Down to the home stretch
Down to the wire
Betting the Chalk/Chalk horse - When a horse is the favorite -- or has the most money bet on it -- that horse is termed the "chalk." Interestingly, this term comes from the pre-computer era of the bookie. When a bookie recorded bets on a blackboard, the odds would change over and over as more and more people bet on the favorite. The horse became known as the "chalk" because the horse's name would disappear in chalk dust as the bookie constantly erased and lowered the horse's odds.
Handicap - in horse racing where originally jockeys had to hold their cap as a early "handicap".
Smart as a whip
In the bag
Falling off the wagon
Beggars can't be choosers
Hell for leather - referred to the terrific beating inflicted upon leather saddles by heavy troopers at full speed, even by Kipling's time it had acquired a figurative sense indicating great speed, on foot, by vehicle, or by horse
Whoa Nellie - "Whoa, Nellie" was a frequent cry given by Gene Autry's pal, Frog Millhouse (Smiley Burnette), as he tried to stay aboard his horse, Ring-eyed Nellie.
Full tilt/Full pelt - a tilt was a joust, you should tilt at top speed, with maximum energy
I'd rather hold a horse in the rain
Get on the bit
Pick Up The Pace
He could talk the stripes off a zebra
With bells on - goes back to the days before automobile, when it was the custom to deck out with the fanciest harness the horse that drew the carriage for special occasions.
Back the field - This is the designation given to the betting against a particular horse (or small group of horses). If a punter believes a horse is sure to lose he can back all the others - 'the field'.
Gee him up
He made a right mare's nest of that presentation
Home and hosed - "The phrase home and hosed was originally used of a horse which had completed a race, was back in its box, and had been hosed down; thus a horse which is described as being home and hosed during a race is a certain winner - it will be back in its box before the rest of the field has finished."
Sitting on the fence: "undecided, unwilling to take a position, straddling.The term blossomed in 1828 and was probably in use before that.Carl Schurz, insisting on political independence, described his position (according to James Blaine) 'as that of a man sitting on a fence, with clean boots, watching carefully which way he may leap to keep out of the mud.'."
Wild goose chase: A 'wild goose chase' was a chase in which horses followed a lead horse at a set distance, mimicking wild geese flying in formation
Straw horse: any weak argument or proposal that won't hold up to intense scrutiny
Hobby horse: a fixation
Peeping Tom - The name comes from the legend of Lady Godiva's naked ride through the streets of Coventry, in order to persuade her husband to alleviate the harsh taxes on the town's poor. The story goes that the townsfolk agreed not to observe Godiva as she passed by, but that Peeping Tom broke that trust and spied on her.
Win hands down: Jockey slacks the reins and the horse still wins easily, with little effort
Put your best foot forward
Got off on the wrong foot
Hot to trot
Stubborn as a mule
Spur of the moment
Raring to go
Marking hames: the wooden or metal pieces forming the collar on a horse, to which the traces are attached; fig. a mess, in the phrase 'to make a hames of,' to make a mess of (possibly because it is difficult to put the hames on a horse the right way up.
Going flat out: The legs stretch out in front and behind at each pace, and the animal as a whole looks flatter and closer to the ground than when it is going at a gentle speed.