Rapid yellowing models seemed to be most common in 1997 and 1998, but these 2000-01 models obviously have the same issue. This doesn't seem to be a problem with later models, but for those produced between 1997 through 2001, keep them well away from the sun. The usual technique of sun-bleaching is not effective with these models.
Starting in 2004 the forelock of the G2 Shetland Pony began to go a bit limp. This issue became worse over time and current models look very much like they've been hit with a hammer.
The first of the G2 models were released with a small stamp with a 1998 date. In 2000 this stamp was replaced a 1999 one which was larger and the orientation was flipped.
Breyer has released Stablemates in rubber tack without issue, but some times, whether due to paint which isn't completely dry, humidity, and/or heat, the tack causes damage. The paint can be completely lifted away, grooved, or just smeared.
The paint on some of the rapid-yellowing models could go a bit odd, looking pale and slightly green; later models were affected by a similar paint issue specifically those painted chestnut or "yellow", such as this #5601 Appaloosa.
Like the rapid-yellowing models, the paint goes pale and slightly green-toned though, with these later models, the white plastic is unaffected. I have models produced some time between 2006-08 with this issue as well.
While this type of plastic was most common in 1997 during one of Breyer's obvious periods of plastic experimentation, translucent plastic still pops up now and then. I have several examples from the 2000-2006 period. The coloured areas on translucent models often look pearly or just extra shiny.
This translucent #5904 Palomino Pony has a smooshed forehead which means he was likely produced sometime between 2004-06 or may be a Target Special Run #720116 Shetland Pony from 2006. The regular run and special run are identical, the only change is in the run number, though translucents seem to be more common in the special run packaging.