Forgeworld MkIc Deimos Pattern Rhinos are currently sold out. While this may be the case of just being sold out, it is expected that the Horus Heresy model range produced by Games Workshop will be expanded upon. This could mean a new plastic Deimos Pattern Rhino is set for the Horus Heresy range.
http://www.forgeworld.co.uk/en-GB/MKIC-Deimos-Pattern-Rhino
While this was discovered yesterday (thanks for the email Bradley), I have not yet found any additional information on it's fate.