Riding keys
Shift runs, T calls the horse after a fall, F is the one-time boost, and Space jumps. Those four actions are on the Roblox description. The description does not say when F resets.
The stat panel observed on November 8 lists Stamina, Acceleration, Handling, Top Speed in km/h, and Boost Length in seconds. Two screenshots showed Top Speed readings of 56.7 km/h and 56.9 km/h. Those are one player's current stats, not a cap.
Prices do not agree
A community horse list prices Black or White, Brown Sugar, Cream, El Conodor Pasa, and Slow Dancer at $2,000 each, and marks Default, Black Betty, Ghost Rider in the Sky, Valkyrie, and Camel as free. The Brown Sugar row on that list was mislabeled with another name, so treat the names carefully.
A November 8 menu screenshot shows Cream selected as 4 of 8 with a Total Price of $8,000 and a Purchase button. That image does not price the other seven horses and does not show a completed purchase. It also does not prove that buying Cream changes your stats.
Read the Purchase button in your own menu. Do not budget every mount at $2,000 after that $8,000 Cream reading, and do not assume every listed free horse is still free.
Rerolls
A November 8 Identity panel shows a $1,000 button when Rolls Left is 0. A second panel from the same day shows 8 rolls remaining and a Roll button instead of a dollar price. Neither image shows a second purchase, so they do not prove that the fee stays flat or climbs.
A developer reply lists horse rerolls among things money can buy, without a fee or a menu path. The $1,000 figure is the button in that one screenshot, not a verified price for buying a named horse. The Mounts tab was not in that crop.
Sandman and horse stats
A helper answer says horse stats also apply to Sandman. It gives no conversion formula. A community horse page says every character can ride except Sandman. Both can be true if Sandman moves without a mount while still using those stats. The November 8 movement note says his boost now starts and ends immediately, his stamina was reduced by 10%, and downhill speed was reduced to bring it closer to horses. No exact downhill speed was given, and that post did not state a server-rollout limit.
Stable and slipstream
The D4C release notes limit the lobby stable area to 10 people at a time. That is occupancy, not a cap on how many horses you can own.
Gyro's first objective is a 15-second slipstream. The FAQ does not give a drafting distance or say whether the 15 seconds must be uninterrupted.
Admin and unconfirmed mounts
The community list marks Limousin Silver Bullet and John as admin exclusive. It marks Night Prowler and the Skull saddle as game passes. The live pass list for this experience does not contain those two names, so their obtainment is unconfirmed. Default, Bounty Hunter, and King's saddles are listed as free on that same page.
There is no published best horse. Compare the five stats on your panel instead of a ranking.