this is not easy to answer! there are a lot of grey areas.
a "nirvana pony" is -almost- anything that was not made in hong kong or china and does not have hong kong or china stamped on the foot. however there are some exceptions to the rules.
when the term "nirvana" was coined not that much was known about ponies and some ponies got put under the nirvana umbrella that aren't really nirvana ponies, when you think about the hoof mark. the german and dutch EXCLUSIVE ponies are a good example of this
they are similar to the UK mountain boys, they were made in hong kong (or china for the dutch ponies) but were only released in one country or small area, they are an exclusive.
the following ponies are all either marked hong kong or china and are just like a "regular" hong kong or china pony (hair, body, eyes etc), but got released in a specific country or region ~ alternate birthflowers, german, dutch and scandanavia.
princess variants and happy tails variants are all marked china i think
and has the normal features of a hong kong/ china pony with the excpetion that everyone who worked in the factory had been to the pub and put them together wrong
thailand is an interesting one! the regular release fancy pants babies got made in either china and thailand
one verson got a stripe/coloured forelock and the other didn't. however thailand is normal for these babies. what is not normal are the thailand mountain boys! they have been classed as nirvana since only a handful have been found and we really don't know much about them.
love pkw xxx
ETA!
i forgot to say the nirvana gallery is back and all the countires as well as oddities are listed there
http://mlparena.com/gallery2/main.php?g2_view=core.ShowItem&g2_itemId=20319 plus ladyg's post on "what is nirvana" is very helpful too
http://mlparena.com/mlp/index.php/topic,283451.0.html