Susan was born to a single mother. This is the entry from GlosBMD:
Child Surname Child Forename Father Surname Mother Surname Mother's Former Name Year District Office Register Entry
THORN Susan THORN 1838 Forest of Dean Westbury on Severn, Newnham 1 257

and there is a transcript of her baptism on this site.

Mary Ann married James Hyett in January 1851. Full transcript on this site:
Groom Surname Groom Forename Bride Surname Bride Forename District Parish Building Year Register Entry
HYETT James THORN Mary Ann Forest of Dean Littledean St Ethelbert 1841 1 14

This is the 1851 Census (Ancestry transcript):
Oldcroft
House Number Given Name Surname Relationship Age Estimated Birth Year Gender Birth Place
65 James Heyett Head 34 1817 Male Oldcroft, Gloucestershire, England
65 Mary Ann Heyett Wife 31 1820 Female Oldcroft, Gloucestershire, England
65 Susan Thorn Relative 12 1839 Female Oldcroft, Gloucestershire, England

and this is the 1861:
Oldcroft
Household Number Given Name Surname Relationship Age Estimated Birth Year Gender Birth City Birth County Birth Country
58 James Hyett Head 45 1816 Male West Dean Gloucestershire England
58 Mary Ann Hyett Wife 42 1819 Female West Dean Gloucestershire England
58 Susan Hyett Daughter 22 1839 Female West Dean Gloucestershire England
58 Richard Hyett Son 13 1848 Male West Dean Gloucestershire England
58 Noah Hyett Son 10 1851 Male West Dean Gloucestershire England
58 Hannah Hyett Daughter 6 1855 Female West Dean Gloucestershire England
58 Rhoda Hyett Daughter 1/12 1861 Female West Dean Gloucestershire England

And should you not have it this is John Hatton's baptism (Source Mongenes):
Parish or Chapel Llanishen
Date 25 Jan 1835
Parent's Surname HATTON
Child's forename(s) John
Father's forename(s) John
Mother's forename(s) Mary
Mother's surname (if different)
Type of Event Baptism
Residence Llanishen
(Father's) Occupation Labourer
Officiating Minister
Memoranda in register
Transcriber's Notes
Gwent record office
register ref. number LDS/PA 46
Parish register entry no.
Parish register page no.
Record_ID 113471
Transcription Llanishen Baptisms 1826-1848

Mike

Hello Karaka,Mary Ann Thorn is my 3rd Great Grandmother you will also find Mary's sister's Sarah born 1816 and Dinah born 1825 also had children before they got married.

Just to add to my earlier post. Susan is listed as a Daughter of John and Sarah in the 1841 Census:

Given Name Surname Age Estimated Birth Year Gender Birth City Birth County Birth Country
John Thorn 62 1779 Male Gloucestershire England
Sarah Thorn 50 1791 Female Gloucestershire England
John Thorn 19 1822 Male Gloucestershire England
William Thorn 17 1824 Male Gloucestershire England
Dinah Thorn 15 1826 Female Gloucestershire England
Richard Thorn 11 1830 Male Gloucestershire England
Lucy Thorn 9 1832 Female Gloucestershire England
George Thorn 5 1836 Male Gloucestershire England
Susan Thorn 2 1839 Female Gloucestershire England

Mike