A "beta monkey" pet; a really cool tiger mount (don't remember what it was for - before my time).

One which I think is an anomaly/oversight rather than an intentional rarity is the Dapple Grey Palfrey horse, which was only available in the crown store from March to May 2015. It was never advertised as rare and cost the same as the other horses, but one day it was removed from the store and has never been available since.

Second to that is the Grey Yokudan Charger which was in the same position, but was briefly available again a few months ago.

I love that several people have mentioned the tiger mount but no one mentioned the other 3 loyalty rewards: the High Hrothgar Wraith pet, Loyal Dwarven Sphere pet and Mask of Cheerful Slaughter.

Those were actually more limited because the tiger mount only required you to have paid for 300 days of subscription time by it's release date, whereas the others were given out when your total subscription time reached 3, 6 or 9 months and by the time the switch to buy-to-play and with it the end of the loyalty program was announced there wasn't enough time to do that if you hadn't already been playing for months.

Oddly it lead to ESO time cards spiking in price and selling out in many places right after it was announced they'd soon be useless, which would normally stop people buying them. People were buying them up purely to make sure they qualified for the tiger mount. (I wonder if that's part of why ZOS did it, then they didn't have to refund retailers for unsold stock they'd just made worthless.)

It was a necessary concession because between the free month at the start, free time given as compensation for bugs and time the servers were offline, and people taking breaks waiting for the game to be fixed a lot of people who had been as loyal as ZOS could expect during the first year wouldn't have qualified otherwise (that's also why it only required 300 days instead of the full 12 months, there wasn't enough time for anyone to reach that). But it did also create an opening for even brand-new players to get it by buying enough subscription time in advance.

(I admit I considered it, but I'd only just decided it was worth getting the game at all, I didn't want to spend over £100 on one mount. I don't regret that, I genuinely prefer the clouded leopard-senche anyway.)

There's also the Vermillion Scuttler pet which was part of then Explorer's Pack pre-order bonus. Oddly I do have that one because even though I bought the game in February 2015 I got sent a pre-order copy and the code still worked, but apparently they were disabled at some point later so the pet is unobtainable now.

I think there were also some in-game pets which came with real-life cuddly toys from the Bethesda store, but I can't remember which ones.