There is a will transcript on Brian Lawson's index for Finloe Cretney 1682: www.iomfhs.im/lawsons/twill/1682_003.html
which mentions his son Oats Crettney "which he had formerly given him in his Contract Bargain". This would be well worth checking (or have checked) in the MM, to see whether both parents' names are given as well as Oats's wife-to-be.

Having written that, I wouldn't believe any of the family lines or dates on ancestry.com unless they give original sources which can be double-checked. There are many errors and guesses which are then copied by other people and republished on other websites.
Where, for example, are the early birth dates recorded?

I can't find a entry in the Maughold parish register for the burial of any Cretney or Christian on or near 20 Dec 1694. No Maughold marriage Cretney/Christian (only 4 marriage entries in the par reg before 1652).

However Joyce Oates wrote a will summary: "Archd will 1689 Marown, of Christian Cretney als Fargher: died 20 Dec 1689: husb Oates Cretney; son William Cretney; dau Jane Cretney; 3 children Phinlo & John & Jane Cretney exec (all underage)." [i.e. under aged 14.]
This will looks like the original source with its details muddled up on your Ancestry record.
Pre-1700 most women were recorded in wills under their maiden names. For this one see the ref: FARGHER, Christian 1689-1 Mar CRETNY A w 0106205.
This and Joyce's other will summaries would be on Manx Notebook.

Sue