Here’s a Reuters story about attempts to bring back aurochsen and introduce them to Britain. Aurochsen are now extinct in the wild, the last having died in 1627. But the Nazis tried to bring them back:
The herd has Herman Goering, the head of Hitler's Luftwaffe, to thank for its existence. Goering hoped to recreate a primeval Aryan wilderness in the conquered territories of Eastern Europe. Two zoologist brothers, Lutz and Heinz Heck, took on the task of scouring Europe for the most primitive breeds of cattle they could find in the belief that by "back breeding" they could resurrect the extinct species.
Heinz Heck, based at Munich Zoo, cross-bred shaggy Highland cattle with animals from Corsica and Hungary, while his brother in Berlin was crossing Spanish and French fighting bulls. The success of the Hecks' breeding program is as disputed as the techniques they used.
The story is about the translocation of some of the animals into southwest England. The Guardian also has a story, naturally titled, “Nazi ‘super-cows’ shipped to Devon farm”:
[F]armer and conservationist Derek Gow has imported 13 of the animals from Belgium to Broadwoodwidger, on the Devon-Cornwall border, where they have joined a growing collection of beavers, polecats and water voles.
Rather than allowing his Heck cattle to be hunted, as some of the Nazi leaders wanted to do, Gow will be offering photographers the chance to take pictures of the animals.
Well, if they were just selecting far-separated breeds for aurochsen-looking phenotypes, they might look like the real thing, but genetically they’ll be somewhat different. But I wouldn’t rule out the possibility of back-breeding the genetics to look reasonably like some wild aurochsen. Old breeds were selected for diverse things, but most of this selection would have used standing variation initially. Few new mutations would have fixed in the time since domestication, and if one fixed in a single breed, it would have been unlikely to have been introduced into other old breeds until recently. So bringing together old breeds from different parts of Europe is perfectly reasonable; much of the old aurochsen variation is still there in different proportions.
The trick is making a population with gene frequencies near those of the wild ancestors. You don’t have to aim for an ideal, since the wild aurochsen would themselves have varied. But if you want to get serious about it, what you’d want to do is complete genome sequencing of many aurochsen skeletal remains.