When picking code names for new software versions, generally you want to stay away from anything that implies mythological or "often mentioned, never seen" status -- users may get the wrong idea. That said, given the long wait that loyal Retrospect users have had for new developments on the Mac side from EMC Insignia (formerly Dantz), I'll forgive the unfortunate choice of "Unicorn" for the beta of the Retrospect client for Mac, and just get busy with the downloading (beta program signup required).
The new client beta, which also requires a patched Retrospect application, is a Universal Binary supporting Tiger and Leopard. In the run up to the Macworld-announced Retrospect X revision of the backup app (anticipated to offer both performance enhancements and feature parity with the Windows versions of Retrospect), EMC will be rolling out additional betas of both the client and application over the next few months.
Retrospect may have lost as many Mac installs as it's gained Windows adherents over the past few years, with frustration over the slow pace of innovation and compatibility on the Mac side leading users to seek alternative backup tools. Unfortunately, for cross-platform small to medium-business backup, Retro is still a leading option. With the forthcoming Retrospect X, at least the loyal Mac customers (and maybe a few new ones) may find a pot of data at the end of the rainbow.