With heavy favorite Rumplestiltskin at the back of the 13-horse field in the one-mile straight event, Fenton kept his 10-1 shot filly at the front on the rail and she kicked ahead just past the halfway mark.
The rest of the field failed to respond and Speciosa finished clear of Confidential Lady in second place and Nasheej in third.
"It's just marvelous. I hope it's given all the little people hope," said Sly, who trained only 10 flat racing winners last season. "Don't give up, you can do it. I'm just overawed really."The Irish-bred filly is by Danehill Dancer-Specufucally (Sky Classic).
Godolphin's Silca Siser finished fourth, followed by Wake Up Maggie (5), Spinning Queen (6), Rumplestiltskin (7), Race For The Stars (8), La Chunga (9), Flashy Wings (10), Nannina (11), Alexander Alliance (12) and Donna Blini.