As an equine humane investigator, I really urge everyone to take a DEEP breath and calm down for a minute.
Deluging relevant agencies with calls from across the country from hysterical people is not going to get anything accomplished.
Lynnwood - Contacting an equine law attorney who will know all the relevant case law before doing anything is probably most advisable before doing anything.
I would strongly recommend taking everything you have on this horse - photos, vet records and especially the contract with the right of first refusal - and visit the retirement facility IN PERSON. Dress professionally, and speak calmly and gently. Attacking these folks will result in defensiveness (at the least) and removal for trespassing + a no contact order (at the worst).
Don’t start off with “he looks like shit - WTF are you people doing to him?!” Instead, thank them for taking him in during his time of need. Learn a little bit about their operation (and subtly try to get a handle on their succession plan - who/what is in place for 130 horses if one or both of them die suddenly?); ask them to share a story about their favorite or most memorable horses.
Segueway into talking about your horse. Share the photos and stories of his youth. Explain how he came to leave your possession, and if it comes up, show the copy of the contract that clearly has ROFR in it. Ask them to understand that you were never contacted to take Max back, which you would have done in a half a heartbeat, given the opportunity.
And you’re hoping with all your heart, that they would be willing to give you that opportunity today.
Having a cashier’s check ready for whatever amount you can/are willing to put into this transaction couldn’t hurt.
Any retirement facility worth their 501©3 would understand what happened and would rather see the horse with its original family. Plus, they are actually opening up a spot to save another horse (or 2) by allowing you to take Max back. Win-win for everyone.
Offer to share your story, and how willingly and graciously they were willing to work with you to get your horse back home, with the local news media. Everyone loves a warm, fuzzy story, and it’s getting to the end of the year = tax deductible donations need to be made + wouldn’t it be a great idea to make a donation to them in the name of someone dear for a Christmas gift, instead of a tchotchke nobody wants? (Don’t offer it unless you really are willing to go on camera on their behalf.)
They’ve gotten a lot of calls - they know this is out there on people’s radars. Everyone know that a negative experience travels 10 times farther and faster than a positive one – and that’s outside of the equine world. In the equine world, your name can become manure faster than a horse can drop it on the ground, and it will take longer to rebuild their good name than it does to grow out any kind of tail on an Appy 
If they refuse to release him, there are two ways to go.
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Offer copies of his vet records, and the care protocol that you followed when you owned him. Ask to speak (or find out) to the vet of record for the retirement facility, and make sure they have copies of his previous records and anhidrosis diagnosis. They are legally bound to prevent unnecessary pain & suffering (under Florida Statute 828.12 (2) http://asci.uvm.edu/equine/law/cruelty/fl_cruel.htm); failing to appropriately care for a horse’s known diagnosis of a recognized medical condition (anhidrosis) should fall under that statute.
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Go down to the sheriff’s office IN PERSON. Dress professionally, and present your case CALMLY. Take everything you have on this horse - photos, vet records and especially the contract with the right of first refusal.
Have a copy of all related equine statutes with you (http://asci.uvm.edu/equine/law/cruelty/fl_cruel.htm) for the state of Florida - chances are, any humane statutes (however weak or strong) are NOT part of the criminal code, but may be found in the Combined Statutes (at least they are in IL), and the sheriff is still responsible for enforcing ALL legal statutes in the state.
However, I will tell you that calling and emailing only proves that there are multiple people willing to give up 10 minutes of their day. Showing up in person, with hard copy proof and being calm, logical and rational takes it to a whole new level. It shows that you are willing to put action and time behind your words.
Regarding the other horses on the site: either it’s dumb luck, or they are purposely shooting the worst horses in a way that you can’t really assess their BCS. Delilah is what I would estimate to be a 2-2.5 on the Henneke scale; Zac is about a 2.5, and Archie is slightly ribby (3-3.5). They cannot be properly scored without laying hands on and feeling for fat deposits in key locations. Winnie was a 1.5 before she arrived; she is at least a 4 (which is normal and perfectly acceptable) now. But … how recent are any of the website’s photos?
Standing in the rescue’s shoes: they take in the horses nobody wants. They probably get multiple calls a day about a particular horse, and then the callers evaporate … for whatever reason. They’ve been doing this for 30 years; they’ve developed thick enough skin to ride this out for a few days/weeks until it blows over. They are older, and have probably reached the “I’ll say and do as I please” stage of life. It is also hard to convince rescue operators that you are not “just some young thing with a couple years of school, a few riding lessons and a piece of paper who thinks she knows about horses”, to paraphrase one of my favorite rescue founders.
Calm, steady, methodical and logical will get you a lot farther in this situation.
He looks like he really needs you. Good luck.