hi sara
i'd be happy to explain a bit for you.
when we do "special cats" we call them "collections" or "collectible". they differ from the normal cats in that they are usually seasonal or for a special event and also sold or available for a limited time.
each time we do a "special" cat or "collection cat", we do it a bit different as far as "how" they are sold (single or in pairs) and what features they will have. until the "fat cats" they were always sold. (except for the "gift" cats, see below) the fat cats were the first in which they could be "born" from any breeding pair of cats. (except for the special 100 leprekittens)
as far as traits, we've done that different each time too, sometimes a trait may be passable, other times the collection cats can hide new traits.
here are some fast facts about them:
1. collection cats are transferable if we sell them.
2. cats which were collectible gifts for no charge like the SL8b birthday cat, the Starry Night cats, were NOT transferable and one per avatar.
3. we sell our collection cats usually for 750L, which includes some food and other things depending on the cat. (1500 for a pair, when sold as pairs)
4. we have done each collection different.
i think the reason that people "chase" them later is because they either want more, or they have become more desirable due to the limited supply, or their popularity has increased overtime.
here is a page where you can see our collections to date and how each was done!
i hope this helps you
http://kittycats.biz/collection-cats.htm
warm regards,
callie cline