Helmet crests?

[Jubal]

So, in images/popular versions at least, it seems that helmet crests were massively A Thing in the ancient world: we usually represent e.g. Hoplites, and Carthaginian and Spanish warriors, with crested helmets, some Roman legionary ranks had them, and so on.

What I want to know is:

• How common actually were they? Would any rank and file hoplite or punic infantryman have actually had a crested helmet?
• What were all those crests made out of? The way they're presented looks like maybe dyed horse hair, or sometimes feathers? Did it matter/was there any consistency?
• Where did they all go? It just doesn't seem to be A Thing in the medieval period, occasional plumed helmets as a bit of a fancy later in the period but we do seem to go from helmet crests being semi-normal in antiquity to just Not A Thing in the period I study, and I don't really understand why this should be the case. Could just be fashion, but it seems odd for something to be a major style in the classical world and then just full-on fall out of fashion for a millennium.

Part of this is sheer curiosity, part of it is that I'm wondering how to do the look of Kesrata and Tabnire in Kavis, much of the theory of which is "Punic cultures but if they still had a couple of big city-states with continuous existence by like the 800s". Thoughts welcome!

[dubsartur]

Right now there is no one good book on Iron Age helmets in the oikumene, just a few with different focuses and blindspots.

Reallexikon der Assyriologie s.v. Helm: Archäologisch (horsehair helmet crests seem to have been invented by the Neo-Hittites then spread to the Assyrians and the Aegean)
Eero Jarva's Archaiologia on Archaic Greek Body Armour
Connolly's Greece and Rome At War (just keep in mind that a lot of his 'classical Greek' kit is from sites in Italy)
Encyclopedia Iranica s.v. Helmet (i) https://iranicaonline.org/articles/helmet-i
Dintisis' Hellenistische Helme (2 volumes)
Bishop and Coulston's Roman Military Equipment

Helmets still have horsehair crests in the Strategikon of Maurice and a silver Lombard bowl. 

Early and high medieval Europe are not my specialty, but by the 13th century the Franks have those cuir boille decorations for the helm, and they last into the 15th century.  They serve the same function of making you look taller and standing out in a sea of screaming moving bodies and flashing weapons.  Codex Manesse and the Innsbrucker Wappenrolle and other MS interested in heraldry tend to show them alongside the shield http://codicon.digitale-sammlungen.de/inventiconCod.icon.%20312%20c.html   By the trecento we also have bascinets fitted with "a little tube of silver to carry the plume" for a few soldi extra.

Claude Blair and David Nicolle are good 'go tos' for medieval arms and armour.

[Jubal]

Thanks, will follow some of those up

And yes, it certainly seems obvious that medieval people had the same want out of helmet decorations re taller-making and fancy - so it's interesting that there wasn't much obvious use of the precursor styles that ancient cultures used to do that.