Radio Times concludes it was Tobias:
In the unfinished footage, fans were treated to a scene set in the home of Benicio del Toro’s Collector, the outer-space character from Thor: The Dark World and Guardians of the Galaxy who spends his time gathering rare species and objects.
In the course of this collecting, we previously saw him get his hands on the Reality stone, which is why he factors into the new film – but eagle-eyed viewers also spotted that one of his specimens in the footage looked an awful lot like an Arrested Development character, specifically David Cross’ never-nude analyst/therapist Tobias Fünke in scenes where he painted himself blue while wearing jean-shorts.
Look, he was trying to join the Blue Man Group, and…it makes sense if you watch the series, OK?
Anyway, the Russos’ comments seem to confirm that this bespectacled blue specimen is an intentional callback to the comedy series (which they previously acknowledged in Captain America: Civil War by including the Bluth’s stair car in the background of a scene), and therefore the first official Easter Egg that we can all bask in the glow of before we see all the rest in cinemas next week.