In 2013 Cool Mini or Not launched what was, at the time, the most successful miniature Kickstarter. to date - Wrath of Kings. One of the adversaries in that were the House of Hadross which had some really awesome miniatures, including some very large beasts.
Knowing the lead time it takes Games Workshop to get an idea into miniature form, 2013 (and the eventual much-delayed miniature delivery in 2015) would make the time they would have been designing the Idoneth Deepkin surely around then... and this very successful Kickstarter surely couldn't have missed their attention!
We're only spit-balling here, but it will be very interesting to see what this release, rumoured to be April, will look like!