Earliest documented fan art of Rainbow Dash with a pet turtle(tortoise) was 8 months ago (March 2011).

This line art was drawn by Madmax.
Probably was posted on FiMchan's fanart board (which Madmax is a frequent contributor of) before it was reuploaded on Ponibooru.
Sometime later the tortoise was being called Speedy.

2 months later (May 4, 2011) a Rainbow Dash toy was released. It came with a scooter, some other stuff, and most noticeably, a baby(?) turtle/tortoise.

1+ hours later an episode aired and introduced Rainbow Dash's pet tortoise that fans have dubbed "Tank".

I'm happy they did, because RD deserves a flying tank a a pet. :)