Wendy

Also Dutch: https://www.meertens.knaw.nl/nvb/naam/is/Wendy

Wendelina could be a longer form. Names ending in '-wen' like, for example, Bronwen, could also work as longer forms.

Also used in French: https://meilleursprenoms.com/popularite-prenom/Wendy

In 2018, 43 is the most common age for an American (U.S.) Wendy* who is registered female with the Social Security Administration. It is the 215th most common female first name for living U.S. citizens.

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*as a first name, not a nickname.

Wendy has indeed been used for boys on rare occasions, but that's usually as a pet form of Wendell. One example was Wendy Bagwell (1925-1996), a famous gospel singer who led the group Wendy Bagwell and the Sunliters:

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https://www.discogs.com/artist/2245523-Wendy-Bagwell-And-The-Sunliters.

In 2014 were born 33 Wendy in Italy (the highest ever).

The name Wendy was given to 328 girls born in the US in 2016.
More than 99.9 percent of people with the first name Wendy are female.

Using the Church of The Latter Day Saints records (familysearch.org) my search has produced the following information.
The name Wendy, is usually female, but is sometimes used for males in the U.K.
The earliest entry for the name found in USA records, is Wendy Pitz born 1796 in Pennsylvania. However, in the U.K. the earliest record is for Wendy Evans born 1469 in Redenhall, Norfolk. There are records for more than 90 names by the year 1900.
(Latter years are death records where recorded.)

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1469 Wendy Evans F Redenhall, Norfolk
1510 Wendy Gail Orr F England
1524 Wendy Ellsworth F Cantebury 1550
1527 Wendy Jones F Yorkshire 1585
1531 Wendy Rice F Cork, Ireland 1587
1540 Wendy Palmer F England
1541 Wendy Greeke F Lanc's
1546 Wendy Talbois F Armagh, Ireland
1555 Wendy Orr F England
1579 Wendy Womack England
1598 Wendy Taylor F Hertford, England 1661
1607 Margaret Wendy Burgoyne F Haslingfield, Camb's
1615 Wendy Oxford Harston, Camb's
1618 Wendy Howard F Witshire
1636 Wendy Zeller F England
1637 Wendy Phillips F Sussex 1708
1662 Wendy Williams Swarby, Linc's
1682 Wendy Thompson England
1684 Wende Willmut Shalbourne, Wilt's
1698 Wendy Burgoyne Wroxall, Wark's
1703 Wendy Bank F Yorkshire
1705 Hannah Wendy Crocket F Kenmare, Ireland
1709 Wendy Oxford Royston, Hert's
1711 Wendy Fuller All Saints, Camb's
1718 Wendy Victoria Fairchild F Somerset
1734 Charles Wendy Addington M Litchborough, Northants
1739 Thomas Wende Beddoe M Ludlow, Shrop's

Wendy was a very common Dutch name in the 1980s; it reached up to about fifth place on the charts.

I have an ancestor named Wendy who was born in 1839, so obviously it did exist before Peter Pan...

The son of William Lloyd Garrison, (the noted 19th century American abolitionist) was named Wendell Phillips Garrison after a family friend (also an abolitionist), and affectionately known in childhood as "Wendy". Although a son and not a daughter, it is perhaps worth mentioning. Wendell was born in 1840, graduated from Harvard in 1861, and as literary editor he co-founded the magazine, The Nation. He remained at the magazine from 1865 to 1906, dying in 1907. Attribution for his nickname can be found in the biography of his father, "All on Fire", by Henry Mayer, St. Martin's Press, New York, 1998, page 298.