Does the ponytail thing in a knight's helmet have its own name? Or are they just called ponytails?
It is a Plume -
late 14c., "a feather" (especially a large and conspicuous one), from Old French plume "soft feather, down; feather bed," and directly from Latin pluma "a feather, down; the first beard," from PIE root *pleus- "to pluck; a feather, fleece" (cf. Old English fleos "fleece"). Meaning "a long streamer of smoke, etc." is first attested 1878.
The Roman plumes were made from horse hair though. Feathers would not survive long.