The volver creature type had five cards in one expansion in 2001 then was never seen again. What happened to them?
Creature types can be a fickle thing in Magic: The Gathering. Some creatures have dozens, even hundreds of cards. Others are a bit more rare. Beebles, phelddagrifs, and others, however, are more rare, appearing once or twice, usually tied to the plane or set they are in, then never used again.
But there is also an unusual record here. What creature type has been in only one expansion, yet had the most to appear during it? The answer is five with the creature in question being a Volver. All of them were in the 2001 Apocalypse expansion and, more than 25 years later, they have yet to return.
Essentially mutant abominations with one of each color created by Rath and Dominaria fusing together, they all came with the kicker ability. For example, Rakavolver:

And they are all like this

They were all pretty powerful, so why haven't they come back? Phyrexia and other similar planes could easily accommodate them. As it turns out, they were a bit too niche. They were specifically created for a one-time cycle in Apocalypse to help show off the kicker ability, so once that was over, they weren't really needed again.
Plus, volvers, while powerful a few decades ago, are really not that strong in modern day Magic. While legal, volvers are unpopular even in low-power friendly formats like Pauper.
Technically, volvers can come back at any time, but the odds of that happening are extremely unlikely. Developers rarely talk about volvers, and even amongst fans they are rarely mentioned. Most tellingly, in the beeble scale which lists virtually every type of creature, they aren't even listed. They haven't been forgotten by any means. It's just that Magic evolved, and volvers were a creature of their time and place, and that time and place has never really been revisited.

A lot of rare creatures suddenly get resurgences. Bloomburrow, for example, boosted a lot of animal creature card numbers to the point that players can now build things like otter decks. Lorwyn Eclipsed did the same for a lot of creatures that hadn't been in prominence for years. But volvers? They are likely not getting a revisit any time soon.