What are some key points you have done when working out a partnership deal on a horse with a trainer? I have one in training right now, I had talked to the colt starter about partnering on me with her, and he said he wanted to but we never hashed out any details, we are going to talk this week and I am not sure what it should look like; i have $25,000 cash into her between purcahse price and PPE/x-rays; his training is $800/month for starting colts.

would love input and pros/cons :)

If you are going to do it you must have a clear and concise contracct.  Down to every detail you have never thought of.  Everything is all well and good going into an Agreement / Contract and then when it comes down to a dispute and confusion of terms - that is when you realize you left details out of the Agreement / Contract. 

I am the trainer in a contract like this right now.

You need to list every possible expense & their due dates/reimbursement dates. For example, owner is responsible for feed but the trainer picks it up and sends invoice to owner on X date of the month, and that is due by X date of the month. Who takes the horse to the vet in case of an issue, and who is billed for that visit. (this includes maintenance and potential emergencies). The point is, be VERY CLEAR about expectations from both parties, from feed to farrier to entry fees to winnings. This keeps feelings from getting hurt and one party feeling taken advantage of. 

Who gets to ride the horse, who gets to haul the horse, who makes executive decisions on injections, dental care, etc?

I would also include a clause for the potential sale of the horse, you never know what will happen in the future.

The best advice you could ever get is,,,,,,,,,,,,,,,,,,,,,,,DON'T

I'm against it, for either side. 
I can tell you from personal experience, there's no way to make it an equal partnership. The party that's doing the training, care and hauling makes the least amount of money (unless they win A BUNCH). 
Be sure you have an exit strategy and buy out plan (along with the standard details) in writing. Everyone is distracted with who pays for what but doesn't really consider the horse having to leave training or getting sold (which is usually where someone gets hosed).

So 




Don't?

I wouldn't (learned my lesson) . 
I figure they're going to to it anyway so I'll give a little food for thought ;-)

This is very very true. To specify, the ONLY reason I am in a partnership like this as the trainer is because I'm trying to get my name out there and more experience under my belt. I have a full time job, I have my own horses, and I only have this SINGLE horse in "my program" right now. I'm not in it for the money, nor is it my full time job. Just thought I'd clarify!