I am not sure that you have captured the true sense of a Folly there adrienne although 'lack of good sense', being the definition of the words other use, may come close. My home town had a Folly, it was a Toll House that I don't think ever served as such but added character. The other that I know well is on a hill near Bath and is known as Sham Castle being the facade of a castle only, much like the Hollywood sign.

A Folly is indeed an eccentric gesture although I don't pick up on the egocentrism that I would attribute to, say Trump Tower. Obviously they don't appear in the gardens of ordinary people so the comparison you make is a non starter.

Ah, but if he were to live in England, Trump would hire some hermit to live in the ruins of a roman pyramid in the middle of the lake...but only after first gilding the poor sod in gold. And inviting the News of the World.

Actually Adrienne if you want something impractical, fantastical, aimless, obscure and ersatz that is also a a laugh-out-loud joke AND American can I suggest Donald Trumps hair?

Hee

Would Cadillac Ranch or Watts Towers fit in your description? Or was their original intention as art (I'm not so sure about the towers) make them ineligible?

I think those would be really good examples of North American follies if they weren't quite so grand and in public.
Unless that's part of the cultural interpretation of the phenomena? Hmmm.

And just for ink - the devolution of taste...
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But Carhenge (the middle photo) is more related to Stonehenge than to a folly.

Interesting take but I think I'd take issue with Adrienne's definition. A couple of thoughts. British follies were built for many divers reasons but mainly as an expression of fashion e.g. for the picturesque, the Gothick, Chinoiserie, Egyptian, etc in past centuries. Many, including Sham Castle, were built to enhance a view or provide a focal point, so they were not exactly aimless. No more bizarre than placing a birdbath in your flowerbed. They just had more space to play with. A few were even built as projects to employ local labour in times of hardship, so not entirely unaltruistic. Nor were all entirely impractical as they served as picnic spots, bath houses, observatories or look outs, etc. Some were used to disguise practical purposes such as pumping houses or air shafts from underground mine workings. They were ersatz in so far as they were not genuine castles or whatever, but no more so than putting Classical columns on a mansion even though it wasn't in Ancient Athens. There are a few follies still being built but they are essentially a historic phenomenon and not a big part of English, or British, gardening culture now. On the whole they add historic and visual interest to the landscape.

Here is a link that might be useful: Follies

I love that picture of president elect Trump that only goes to demonstrate that nature will have her way.

I bow to the Brits.

Arise Lady Adrienne.