There are so many European released ponies that to include them under the Nirvana umbrella would cause so much crossover with Pony Corral, as most European issue ponies are well known and reasonably available.
Nirvana ponies are primarily those that are made in countries other than China or Hong Kong, regardless of how common they are (for example, Italy Peachy and NC Sundance are both really common, but still count as Nirvanas!). There are a few exceptions to this though, that are included due to specific area releases (eg. Dutch ponies and German ponies), though equally they can be included in Pony Corral also.
Just to confuse the issue, there are very rare variants of Babies Billie and Susie in alternate poses that are considered Nirvana