1991:

SECRET SURPRISE PONIES

The Secret Surprise Ponies came out in 1991-2 in the United Kingdom. I can be pretty sure of this, because I was fortunate enough to get one for my tenth birthday. I wasn't aware of the set at that time, so I suspect they must have come out quite late or did not appear in the comic until quite late (as this was my source for whatever was coming out.)

SECRET SURPRISE PONY KEY/CAPE MECHANISM
(image from Spangling Secret's UK Box)

Built larger than the average pony, they had a pose designed just for them. Possibly inspired by the success of Tonka's Keypers line, these ponies had little compartments hidden under a cape which could be unlocked by a special key that you could hang around the pony's neck when not using it. The keyhole was on the pony's chest. The ponies came with a treasure - two of them had rings and two of them had charm bracelets. If I remember right, their eyes also had a slightly different look - the small white dot of light was either shaped as a heart or as a star. Ponies with a star came with a bracelet, and those with a heart came with a ring.

The capes can be detached at the hinge, and because of this, you can find these ponies second hand without their capes. This is a pretty major flaw and most collectors look for ponies with their capes intact.

The set was sold worldwide, and the set name was the same in North America and other places, but the ponies had different names here from their North American release. Here, all the ponies had the word "Secret" in their name, with an alliterative preceding word relating to jewellery. They were all earth ponies, all with solid single colour mane and tail, but the capes were patterned. Again, the capes were either star or heart patterned (this matched the eye).

SECRET SURPRISE PONIES: UK NAMES
(From Spangling Secret's UK Box)

In the US, they had the following names:
Shining Secret = Stardazzle
Shimmering Secret = Secret Beauty
Sparkling Secret = Secret Star
Spangling Secret = Pretty Puff

This set had a comb which was unique to them - small, chunky with an embossed design on the front (and not much use for combing hair!)