Slang for slow horses.

I live in another country, Brazil. In here, whenever the agents from Slough House are called slow horses, the subtitles translate it to "pangares." It's a slang here for a horse that doesn't look to good, and since calling someone a slow horse (cavalo lento in Portuguese) doesn't make sense to us, it fits. So I've started to wonder if there isn't a slang for slow horses in English. Or is it perfect ok to call someone a slow horse that you guys don't need slang for that expression? Or maybe the show really wants us to know that it is called Slow Horses.