Most that raced will be consummate professionals to some extent; give them time to step out of their box. Sometimes they are shut down, sometimes they have a physical component keeping their personality at bay.
Aggressively address their hooves, which often come with unbalanced angles (NPA) up front and behind. Remove any toe grabs immediately. Consider putting them on a short cycle to get their toes back and heels growing correctly.
Some sort of ulcer regime is good; if not Gastroguard, look at Nexium.
Don’t forget dental; some come from connections that do routine dental, but many don’t. All of my TBs have come off the track with questionable – if any – dental history.
Keep hay in front of them 24/7. They are used to a hay buffet at the track. I start all my let-down TBs on alfalfa pellets with a high fat/high fiber grain.
It would not hurt to have a chiro and vet look at this horse to offer a Day 0 baseline. Take videos, pictures regularly to keep track of progress or any backsliding.
Find a 24/7 turnout situation. This is my ‘non-negotiable’ for my horses.
Their letdown period depends on why they are retiring. Racing is the hardest job there is. Having some contextual understanding of what racing is will help you understand: these horses are juvenile/immature horses, who are fastest their 2-3 y/o year. They are often backed in a stall or small farm before their 2nd birthday. Depending on the farm, they may be started undersaddle by walking around the shedrow, in a round pen, or they may be started with a “nanny” horse who leads them to/from the yard to the track. Some are ponied. They are fed early, worked early, and rubbed down/bathed/wrapped all often before 11 AM. They live in a stall for the remaining 23 hrs a day that they are at the track; most venues do not have turnout or if they do, they are small (2-3 stalls in size). Many are housed in a shedrow style barn, and are handwalked or ridden inside the barn. They are often stalled with a haybag with half a bale’s worth (20-30lb) of hay, filled once in the AM and filled again PM. Many are used to being in the stall while you pick it, fill water, etc. They will typically be exposed to bikes, golf-carts, cars, even scooters or motorbikes.
Because they are worked hard and then put in a stall, many develop soreness over their back and SI because of the hard work and lack of movement. The motion is the lotion with athletes, and these horses are the hardest working athletes there are.
If they are coming off the track they are guaranteed backsore and guaranteed unsound to some extent - either through their work, their stalling, or their race-track hoof angles. Sometimes this resolves of its own; sometimes they have ‘jewelry’ or baggage from the track that require time and therapy to resolve. Make sure to get a PPE and ask more than one person what they think about the horse before you buy it. Picking up a SOUND TB from the track is an art, takes a lot of experience and a special eye to see what is regular old “trackiness” (which is just horse slang for a horse who is tight or snappy over their body) and what is an undiagnosed injury.
The first few weeks is an excellent time to work on ground manners and lunging. Most TBs will come off the track being micromanaged and handled often, so they will know to bathe, single clip, lead, etc. Teach them to ground tie, lunge, cross-tie if they don’t know, etc. The first month or two is a good time to do in hand work and desensitization work: cross tarps, bridges, poles, sack them out with crackly grain bags, do jumping jacks around them, pony them off of a schooled horse buddy, etc. Track their reactivity and disposition - are they worried all the time, do they stop and think?
I work on in hand things like teaching them to move over/leg yield to pressure, teaching them to yield their head if they step on the leadrope, politeness while going through gates - a pet peeve of mine! Work on teaching them to stand at the mounting block, which few know how to do. Remember these horses are trained, but not in the discipline of your choice. Most will pick up and understand pretty quickly.
I usually spend the first month of riding hacking around the property. We might do some light ring work and poles, but my priority is to get their confidence and exposure up.
Avoid an inconsistent routine. They thrive on routine and come from a heavily regimented, routine environment.
Work closely with your trainer. If you have never started a horse or worked with an OTTB before, this is out of your paygrade. Be realistic, and understand that this is a long process that is sometimes as unrewarding as it is rewarding. Read everything you can about restarting track horses, and don’t be afraid to involve other people, too.