
The Athenians never called Hymettus simply "the mountain". They gave it a nickname.
Trellos, "the crazy one". Among the people Hymettus is called Trellos or Trellovouno ("crazy mountain"). Greek Wikipedia notes that the name probably comes from the French phrase très long, "very long", used by foreign visitors. English Wikipedia says the name refers to the mountain's long, winding length. Athens Voice writes that for the Turks Hymettus was Deli Dagh, for the Franks Monte Matto, and for the Greeks "Trelos". All three mean "crazy mountain". Athens Voice gives no source for the claim. The explanation of the name therefore remains conjecture, with three versions that differ: the French très long (Greek Wikipedia), the winding length (English Wikipedia) and the foreign names (Athens Voice).

An old saying. In the past Athenians were called thoroughly mad, "since the Sun rose from Trellos and set at Dafni", as Wikipedia reports it.
Songs. Light songs took Hymettus as their theme, such as "Up high on Hymettus there is a secret".
The monastery and letters. The Ministry of Culture writes that Kaisariani Monastery was for many years financially sound and had a notable library. The Ephorate of Antiquities adds that its prosperity rested on olive growing and beekeeping. We do not repeat the claim that the library held archives from ancient libraries. It appears on a Philodasiki page without a citation. The monastery preserves the Tower of the Benizelos family, a three-storey building. In post-Byzantine times part of the complex served as an olive press.

Kazantzakis and the Monastery of the Hunter. In the 1920s Nikos Kazantzakis intended to found a spiritual community at the Monastery of Saint John the Hunter. The information comes from Greek Wikipedia, which cites the Kazantzakis and Lambridis correspondence (2018 edition, p. 21). We did not open that book.
The lion. Local tradition says a lion once lived on the mountain and terrified the people around it. The Liontari (Lion) cave takes its name from it. According to the Ministry of Culture it lies on the eastern side of the mountain, at 550 metres, west of Glyka Nera. Wikipedia places it in the north of the mountain. According to the tradition recorded by the Ministry of Culture, the lion terrorised the inhabitants until it finally turned to marble. A marble statue of a lion stood near the church of Saint Nicholas in Kantza until the Second World War. This is tradition, not historical fact.
What was found in the cave. The cave shows traces of use from the Neolithic, from the early 5th to roughly the end of the 4th millennium BC. In Classical times and later it served as a sanctuary, most probably dedicated to Pan and the Nymphs. In modern times it was used as an animal shelter. It is not an organised archaeological site. It measures about 50 by 20 metres, with a ceiling height of 6 to 11 metres.
Not verified: (1) The etymology of "Trellos" is a guess ("probably"), and the two Wikipedias give slightly different explanations. (2) No source was found for the age of the saying, the song or its author. (3) The Myths and legends page is not covered here, and the mountain's culture deserves primary research (folklore collections, archives). If you have your own story or song, it can be added.
Sources
- Wikipedia (Greek), Υμηττός, Folklore section: https://el.wikipedia.org/wiki/Υμηττός
- Wikipedia, Hymettus: https://en.wikipedia.org/wiki/Mount_Hymettus
- Ministry of Tourism, Odysseus, Lion cave of Hymettus: http://odysseus.culture.gr/h/2/gh251.jsp?obj_id=16861
- Athens Voice, Hymettus, the most Athenian mountain (08/06/2022): https://www.athensvoice.gr/life/life-in-athens/761073/ymittos-pio-athinaiko-voyno/
- Ministry of Culture, Odysseus, Kaisariani Monastery: http://odysseus.culture.gr/h/2/gh251.jsp?obj_id=1524
- Ephorate of Antiquities of East Attica, Kaisariani Monastery: https://www.efaanat.gr/index.php/portfolio/moni-kaisarianis/?portfolioCats=46%2C38%2C39%2C40
- Wikipedia (Greek), Monastery of Saint John the Hunter: https://el.wikipedia.org/wiki/Μονή_του_Άγιου_Ιωάννη_του_Κυνηγού