Yngvi, Ingui or Ing appears to have been the older name for the god Freyr, which meant "lord".

In Scandinavian mythology, Yngvi, alternatively Yngve, was the progenitor of the Yngling lineage, a legendary dynasty of Swedish kings from whom the earliest historical Norwegian kings in turn claimed to be descended, see also Freyr.

Information on Yngvi varies in different traditions as follows:

• Yngvi is a name of the god Frey, perhaps intended as Frey's true name while Frey 'Lord' is his common title. In the Ynglinga saga and in Gesta Danorum, Frey is euhemerized as a king of Sweden. In the Ynglinga saga, Yngvi-Frey reigned in succession to his father Njörd who in turn succeeded Odin. Yngvi-Frey's descendants were the Ynglings.
  
  
• In the Islendíngabók Yngvi Tyrkja konungr 'Yngvi king of Turkey' appears as father of Njörd who in turn is the father of Yngvi-Frey, the ancestor of the Ynglings.
  
  
• In the introduction to Snorri Sturluson's Edda Snorri claims again that Odin reigned in Sweden and relates: "Odin had with him one of his sons called Yngvi, who was king in Sweden after him; and those houses come from him that are named Ynglings." Snorri here does not identify Yngvi and Frey though Frey occasionally appears elsewhere as son of a Odin instead of a son of Njörd. See Sons of Odin.
  
  
• In the Skáldskaparmál section of Snorri Sturluson's Edda Snorri brings in the ancient king Halfdan the Old who is the father of nine sons whose names are all words meaning 'king' or 'lord' in Old Norse and nine other sons who are the forefathers of various royal lineages, including "Yngvi, from whom the Ynglings are descended". But rather oddly Snorri immediately follows this with information on what should be four other personages who were not sons of Halfdan but who also fathered dynasties and names the first of these as "Yngvi, from whom the Ynglings are descended". In the related account in the Ættartolur ('Genealogies') attached to Hversu Noregr byggdist, the name Skelfir appears instead of Yngvi in the list of Halfdan's sons. For more details see Scylfing
  
  

(The Yngling Saga section of Snorri Sturluson's Heimskringla also introduces a second Yngvi son of Alrek who is a descendant of Yngvi-Frey and who shared the Swedish kingship with his brother Álf. See Yngvi and Alf)

Jacob Grimm in his Teutonic Mythology and many others have considered it likely that Yngvi was originally identical to Ing/Ingo/Ingui, the legendary eponymous ancestor of the Ingvaeones.

The element Ing- in Old English names is usually considered to be related.

The Old English Runic Poem contains these obscure lines:

Ing wæs ærest mid Eástdenum

gesewen secgum, oð he síððan eást

ofer wæg gewát. wæn æfter ran.

þus Heardingas þone hæle nemdon.

Ing was first amidst the East Danes

so seen, until he went eastward

over the sea. His wagon ran after.

Thus the Heardings named that hero.