Well, the international (European) exclusives are:
- Star Swirl
- Rainbow Flash
- Daisy Dreams (also released as a TRU exclusive, but without a forelock)
- Ploomette (non-glimmerwing version)
- Skywishes (non-cutie mark magic version)
- Sweetie Swirl
Minty is also quite rare as she was only released in the Costco exclusive 'Midnight in Canterlot' set.
I'm not sure if the ponies from the Canterlot line such as Starbeam Twinkle, Shinebright Luna or regular (non-princess-sized) Luna also fall into that catagory - they were target exclusives which I don't think ever made it to Europe. Not to the UK at least.
Holly Dash and Ribbon Hair Starlight Glimmer were supposed to be regular releases but the distribution didn't go as planned so they weren't available in every country.
There's also Nurse RedHeart. She's a USA walgreens exclusive.
... And then there's a whole category of G4 ponies which fetch high prices because they're 'sought after', such as the first waves of ponies and
Pony (Ditzy Do/Bubbles, whatever you want to call her). They all had regular distribution (I think) but aren't in stores any more and most of them only had one or two releases. I wouldn't say they're 'rare' exactly, but they are in high demand. Most G4's (except the mane 6) go into high demand after they leave stores.
The only other one I can think of is the prototype of brushable Sweetie Blue, but I don't think she's the level of rarity you're looking for - I've only seen her on ebay once and the seller wanted a starting bid of roughly £1,500....