I must admit that I haven't had an awful lot of luck with that website. Seems OK for TBs but not a lot of good for other breeds. Perhaps I'm doing something wrong.
Does it work like a database for ALL horses and ponies with passports and known parentage?
I find allbreed absolutely invaluable for QH's, Paints, Appaloosas, Canadians ..... actually every breed I own has been quite easy to find the pedigrees.
Are you sure you are entering the names in correctly? I know that sounds silly but sometimes the person who inputs a horses name in makes mistakes or misses bits out. If not then you can start the line up yourself and eventually you should be able to work back to a horse which is already in there.
They have all been British Bred horses and ponies which have been registered with their various breed societies. I don't think allbreedpedigree.com covers British pony breeding as well as American or Canadian breeding.
I noticed that the website invites people to enter their own horse's breeding but I can't do so because I need more information from them!!!