They're still in the background. They were a slave-race of the Tyranids who showed up early trying to escape the Hive Mindand ended up being destroyed in the Zorastra-Attilla Wars. They have been designated Hive Fleet Colossus. Most of this comes from the 4th ed Tyranids book, but 5th ed also mentioned the fleet. There was even a shout-out to them in a WD with a make-your-own-monster article that pretty well sealed the deal as to what the studio was thinking about.



SIlly as in the drawing that has been posted earlier in the topic 