I have exactly this same plan but will be on my way from Panama to Mexico so maybe we can meet up on the way somewhere in the middle to exchange experiences.
oh and i forgot to add that i have my ticket all ready for 3rd of october
Alex, I assume this is your first trip? Having a dog sounds romantic and if he's a little guy it's a great way to meet chicks, but consider the border crossing hassles with inoculations/shots and the veterinary inspection paperwork, not to mention the multitude of hotels/hostels that won't accept you, the public transport that won't sell you a ticket, being on the constant look-out for feral local dogs who are scrapping for a fight, etc. Think about it.
Think about the horse plan too... do you know enough about horses that you could buy one and take care of it and the tack properly until you finally reach another destination where you'll be able to sell it?