The name has many variations and derivations, mainly as a result of the difficulty for non-Welsh speakers of representing the sound of the initial double ll (a voiceless alveolar lateral fricative).

[1] A number of other variants have arisen, however, including Elilevelin, Ffuellen, Ffuellin, Fflellen, Flawelling, Fleuellen, Flewellin, Flewellen, Flewelling, Flewellyn, Fluellen, Fluellin, Fluelling, Flwellin, Fowellen, Fuelling, Lawellins, Lawellen, Lewellen, Leoloni, Lewallen, Lewlin, Lewilin, Llallin, Lleulin, Lleulini, Llewen, Leuleijon, Llewelling, Llewellinge, Llewellen, Llewhellin, Llewhelyn, Llewillin, Lluellen, Luellen, Thewell, Thewelinus, Thellyn, Thelen, Thewelling, Thelwelin, Thlewelyn, and Swellin.

In some texts no attempt is made to find a way of writing the ll sound and it is left as l.An alternative strategy was to substitute a similar-sounding name.

Apart from variants mentioned above, surnames originating from Llywelyn include Apswellen, Bewellin, Bewlyn, Bewlen, Pawelin, Weallins, Wellings, Wellins, Welyn, Wellyns, Wellens, Wellence, Wellon, and Wellys.

In some regions of England the surname Welling may derive from an English place-name, but in Welsh Marcher counties its frequency suggests that it can be traced back to (Lly)welyn.