https://economistvision.com/actualidad/costa-rica-probara-en-pacientes-con-covid-19-suero-producido-de-caballos-inmunizados/

Article is in English.

I think I understand this, but not really sure I do. Could something like this be viable as a treatment/vaccine?

In those smaller countries with legal systems that won't allow humongous lawsuits the testing of these vaccines and other concoctions should be able to be done pretty widely and inexpensively. I wonder if that is being done.

I would be interested in the resident MD's opinion on this, it is quackery or potentially viable?

Well, it isn't a vaccine, so the picture is misleading. Rather it is an antibody treatment, just using horses rather than recovered humans.

It isn't crazy. Many snake anti venoms are produced this way. You can scale production more easily than blood donations from recovered people, especially in countries with a less than robust blood supply.

In the US, you see a higher tech approach with monoclonal antibodies. Find a few antibodies that work really well and mass produce them. More consistent product, plus all the exclusivity benefits that make developing the product worthwhile.

I'm pretty sure they used horses to develop a vaccine for the Spanis Flu in the 1920's. It's not a new concept.