Why do people think that G1 Firefly is Rainbow Dash's mother?

[The Nth Doctor 652]

But before they started introducing parents and making them boring, speculation was always fun

Also not helped by Twilight Velvet having G1 Twilight's colors (there's also a pegasus Twilight, but no one ever remembers her)

(although Whizzer is way more Rainbow Dash than Firefly was - she's impatient and a fast flier that doesn't like waiting around)