Anything, absolutely anything, can be forgotten.

I received a comment once, joking about how Wales has likely lost a few mountains along the way, and well, they were right.
Names are impermanent, no matter the physical size, or fame, of the object being named. Welsh history knows this all too well, and even mountains are not immune to being lost, and in many cases shown here, they have had their names lost perhaps permanently.

King Arthur:    • King Arthur: What Everybody Gets Wrong  

Chapters:
0:00 - Ring Ring
0:47 - Forgotten
2:08 - Unknown
3:37 - Missing
5:03 - Replaced
8:09 - Vanished
10:11 - Renamed

Sources (turn on captions):
Bartrum, P.C. (1993). A Welsh Classical Dictionary: People in History and Legend up to about A.D. 1000. The National Library of Wales.
[1] p.29,
[2] 484.

Bromwich, R. (2014). Trioedd Ynys Prydein. 4th ed. University of Wales Press.
[3] p.97.

Gunzburg, D. and Brady, B. (2021). Space, Place and Religious Landscapes: Living Mountains. Bloomsbury Publishing.
[4] p.176,
[5] 177.

Lloyd, J. E., (1959). Dictionary of Welsh Biography.
[6] DAFYDD ab OWAIN GWYNEDD (died 1203), king of Gwynedd.

Owen, H.W. and Morgan, R. (2007). Dictionary of the Place-Names of Wales. Llandysul: Gomer Press.
[7] p.38,
[8] 45,
[9] 72-73,
[10] 91,
[11] 146,
[12] 422.

Pierce, T. J., (1959). Dictionary of Welsh Biography.
[13] DAFYDD (DAVID) ap GRUFFYDD (died 1283), prince of Gwynedd.
[14] DAFYDD ap LLYWELYN (died 1246), prince.
[15] LLYWELYN ap GRUFFYDD ('Llywelyn the Last,' or Llywelyn II), Prince of Wales (died 1282).
[16] LLYWELYN ap IORWERTH (or 'Llywelyn the Great', often styled 'Llywelyn I', though in strictness the first prince of that name was Llywelyn ap Seisyll; 1173-1240, prince of Gwynedd.

Snowdon Experts (2022). Snowdon Stats.
[17] https://snowdonexperts.uk/snowdon-stats/.

Primary:
Cliffe, C.F. (1848). The book of South Wales, the Bristol Channel, Monmouthshire, and the Wye. London: Hamilton.
[18] p.80.

Etymology of Rhyl. (1905). Rhyl Journal.
[19] https://newspapers.library.wales/view...

Nicholson, G. (1840). Nicholson’s Cambrian Traveller’s Guide. 3rd ed. London: Longman, Orme, Brown, Green & Longmans.
[20] p.537.

of Llancarvan, C. (1832). The History of Wales. Shrewsbury: John Eddowes.
[21] p.12.

Skrine, H. (1798). Two Successive Tours Throughout the Whole of Wales. London: Elmsley and Bremner,
[22] p.32.

Redfield, J.S. (1838). Tour in Wales. The Penny Magazine of the Society for the Diffusion of Useful Knowledge, 7-8.
[23] p.167.

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Music courtesy of the YouTube Audio Library:
Doctor Momentum - Slynk
Hypnosis - Godmode
Fast Times - Quincas Moreira
Creep - Emmit Fenn
Torture - Coyote Hearing
Blast From The Past - Jeremy Black
Light-Gazing - Andrew Langdon
Heartbeat - Godmode
Stuck In The Air - The Tower of Light
Fortress Europe - Dan Bodan

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Images of, and from:
Cnicht From the Cob, Porthmadog, Gwynedd by Peter Trimming, CC BY-SA 2.0 https://creativecommons.org/licenses/... , via Wikimedia Commons
all other images are public domain, via the Yale Center for British Art, the National Library of Wales, and the British Library