Horses and dogs are different - I mean, obviously 😂 but not in a facetious way.
If you’re looking for a horse just to be a happy hacker, you’re looking at horses that are already at that point, it almost doesn’t matter what breeds are in it because it’ll be an adult, trained and vet checked. It’s lineage definitely doesn’t matter… and it’s normal for horses to be sold on multiple times as peoples needs or circumstances change, very few people can spend horse upkeep money on a pet horse and it’s not in a horses best interest to be retired at 10 because someone outgrew it.
A dog obviously would be with their owners their whole life barring huge life changing events and then responsible owners would be returning them to their breeder or using a rescue, not just selling them.
Good breeders taking a dog back will either just keep it, rehome it to someone they know or use the breed rescue to look for a new owner, so again, not selling it.
It’s fairly unlikely you’d just stumble on a good breeder with an adult dog to rehome, so realistically the place to get an adult dog is a rescue… in which case, yep it matters not a lot what pedigree it has or hasn’t got as you’re looking at an adult dog in front of you.
Puppies though, are an unknown quantity - so pedigrees matter because you’re having to base your decision on breed traits it’s not showing yet, so to get those breed traits you want it to be a good example of that breed and with a pedigree you can check yourself stuff like health test results, inbreeding, it’s relatives health test results.
Buying a puppy from the adverts you’ve been posting is a bit like you want a highland or a cobby type around 14 hands and someone going, I have one, do you want it? No other info, no pictures or video, no vet checking…, yeah there’s a slight chance it’s what you’re looking for, but it’s more likely there’s a health issue being hidden or it arrived and it’s actually a Shetland