SPOILER WARNING: This article contains major spoilers for "What's Past is Prologue," the 100th episode of The Flash.
Reverse-Flash knows his speedsters. In "What's Past is Prologue," the 100th episode of The Flash, Barry and Nora took a little visit to the past to gather some items with which they hoped to defeat Cicada. Along the way, they bumped into Eobard Thawne, who mistook Nora for a handful of other speedsters: Dawn, Jesse Chambers/Lawrence and Danica Williams.
Dawn, of course, refers to Dawn Allen. Prior to The Flash Season 4 finale, many fans had assumed Nora was Dawn, Barry and Iris' future daughter. In the comics, Barry and Iris had a daughter named Dawn Allen after they retired to the 30th century. Both she and her twin brother Don inherited their father’s speed. They never met their father, who died before their birth during the events of Crisis on Infinite Earths, but soon followed in his footsteps to become Speed Force heroes known as the Tornado Twins.
Fans of The Flash will also be familiar with Jesse Chambers, aka Jesse Quick. On the show, Jesse is the daughter of Earth-2's Harrison Wells. In the comics, however, she is the daughter of speedster Jonathan Chambers, aka Johnny Quick, and Libby "Liberty Belle" Lawrence. Unlike the other speedsters, Jesse Chambers derives her powers from a special formula created by her father, which he passed down to her. Born Jesse Chambers, she once took her mother's last name during a feud with her father.
Finally, Danica Williams is another speedster of the future. She succeeded the mantle of the Flash from Wally West in Justice League Beyond. She hails from the 2040s, just as Nora does, and once worked at the Flash Museum.
It is unclear what significance -- if any -- this holds for the future of The Flash. However, their mention here suggests that they do exist at some point in the Arrowverse's timeline. This could very well set the stage for an appearance down the line, though they're relegated to an Easter egg for now.
Airing Tuesdays at 8 p.m. ET/PT on The CW, The Flash stars Grant Gustin, Candice Patton, Carlos Valdes, Danielle Panabaker, Tom Cavanaugh, Jesse L. Martin, Danielle Nicolet, Hartley Sawyer, and Jessica Parker Kennedy.