What is a good 3D game engine for a horse racing simulation game? Thanks

That's a rather vague question. Maybe you should explain what exactly you are looking for. A lot of 3D engines could make a horse racing simulation.

Jockey's view of the race with the player controlling the assigned horse

Similiar to a 3d car racing game on an oval track

Its according on how much you are willing to spend on an engine. The most sufficient for many genres would be Torque (www.garagegames.com) they offer indie licenses of the engine for $100 and commercial arround $250. Also whats good torque is working into XBOX 360 (Torque 360) altho not released yet i would keep my eye on it :)

I wouldn't bother spending money on an engine if you are willing to put as much effort into your game as you do your posts here.

What sort of effects are you looking for? What level of detail in the graphics? Target system for the game? All these things effect what engine you would want to use.

But really, go open source engine on it, or write you own.

I've become quite fond of the Unreal engine. You could make an Unreal TC mod.