It seems strange that the English name Abergavenny, which looks like a thoroughly Welsh name (albeit phonetically rendered), should not exactly correspond to the actual Welsh name. Was there ever a time when the town was called in Welsh Abergafenni or something of the sort?
Almost certainly! In fact, we sometimes have to explain that we had a recent Eisteddfod Genedlaethol in Abergafenni to people from outside the area who are not familar with the shortened form. The theory seems to be that Y Fenni was used as a shorthand by some local 15th century scribe and has somehow stuck as the 'official' name. There are probably many other places in Wales where the local short form is used locally, but where it has not become the official form, for example:
- Mach - Machynlleth
- Llani - Llanidloes
- Aber - many places beginning with Aber- (as well as those actually called just Aber)
- Y Bermo - Abermaw (Barmouth)
- Y Nant - Nant Gwrtheyrn
- Llandod - Llandrindod (Llandrindod Wells) - The English 'Wells' was probably added in past times, as happened with a number of other places in Britain, to attract tourists
- Llambed - Llanbedr San Steffan (Lampeter)
Official names are sometimes imposed on places that do not want them. For example, Crug Hywel (Crickhowell, in Powys) is the locally preferred traditional name, taken from the name of a fortified hill overlooking the town. More recently it had the phonetic Crucywel allocated to it 'officially', but the local councils seem to stick with their voters' preferred form and have generally ignored the attempted change.
Starting this course in middle age, one of the things I've realised is how most of the Welsh words I already knew were based entirely on 1970s/80s childhood holidays (Caerdydd, Ynys Môn, Ysgol, Heddwas, Toiledau) and reading the regional variations in TV guides (Pobol y Cwm, anyone?)