I was born in Northern Cali. But now I live in Texas where our two main languages are English and, recently, spanish. Here, racial jokes are not taken very seriously. My parents have respected hispanic friends who make wetback jokes. I am surrounded by Illegal immigrants who do not know our laws and treat their horses like replaceable objects. They use illegal spurs and make their horses' mouths bleed with snaffle bits. And of course the police don't do anything about it and humane society can't do anything because it is not in "city limits". It infuriates me. Do not buy a horse from these people. Most of them do not take their horses to vets and most of them don't know what a coggins test is. It is seriously bad down here. It is like the horse abuse capital of the USA.
The problem is that we aren't where you are and a lot of us take offense to blanket statements made in poor taste such as you have made. I lived in Tucson for six years and was married into a Hispanic family for three and a half years. My ex-in-laws may not have spoken great English, but they at least tried...much in the same way the person who wrote the ad you are referring to is trying. At least the ad is written in English, which indicates the person who wrote it at least TRIED to learn the language of the country where they now live.
Also, the things you've cited, such as no coggins tests and no veterinary care, are not a part of Hispanic culture. Most of the people who immigrate could not (and still cannot, in most cases) afford veterinary care. They grew up on farms where a horse was ridden until it was not usable anymore. If it was sick, you put it in a stall and hoped it got better, and if it didn't, well, you had another mount you could use...or you tried home remedies on it. My ex-husband was born in Hermosillo, MX, as were his parents. His dad worked on a family ranch until he married my ex's mom and they immigrated (legally) to the US. When I was married, we talked about getting horses and my father-in-law heard us talking one day. I had mentioned something about shots, farrier costs, coggins tests, vet care, and other incidentals of horse ownership that cost money. He looked at me like I was insane and said that he could trim the horse's hooves himself for free and that was all that was needed.
I'm not saying that all Hispanic people are like my in-laws, because I know they aren't (I'm not that naive). However, to make blanket statements implying that all Hispanic immigrants (illegal or legal) are horse abusers and don't know how to handle a horse properly...and won't learn English...is beyond rude and is borderline racist, regardless of how many people you know who don't take racial jokes seriously.
The point of this whole rant is to watch what you say on the internet. You are on an international forum, not sitting at home among your friends where you can freely make racial slurs and laugh at them. Have a little respect for people who are trying to better their lives by coming to this country.
Also, as an aside, I doubt anyone is considering buying a horse from these people, so your little "warning" is null and void...and I'm not quite sure what the point of it was, to be honest.