Of course, the "HHG2TG" of Toobworld is what I'm always dealing with, which is fine with me; I love it, despite its flaws. First of all, no offense to Geoffrey McGivern, but David Dixon personified the look and personality of Ford Prefect. (If only he had been offered the chance to play the Doctor!)
And it's in Toobworld where the legacy of Douglas Adams has had the best opportunities to establish deep roots into the TV Universe. And that's mostly through simple mentions in a show like 'Doctor Who'.
- We've seen the Fourth Incarnation of the Doctor reading Oolon Colluphid's book, "Origins of the Universe".
- The Tenth Incarnation thought the number "42" might be the security protocol to gain control of the Host about the starship Titanic.
- He also observed that Arthur Dent was a very nice man who also wore a dressing gown when dealing with aliens.
- And then the Eleventh Incarnation of the Doctor actually pointed out a Hooloovoo while he and Clara Oswald were visiting a marketplace on Akhetan.
According to the Doctor, most of the "people" in the marketplace, buyers and sellers alike, were from the local region. This would suggest that the Hooloovoo were from one of the seven worlds in the Akhet solar system.
But the Hooloovoo seen in the marketplace looks nothing like the description from the books: A Hooloovoo is a hyper-intelligent shade of the colour blue. But the alien pointed out by the Doctor was a bipedal humanoid, as seen below:
I think the answer is simple. The Hooloovoo were not so much an intelligent shade of blue, but a blue-shaded intelligence. Much like the Changeling Founders and the Q Continuum, the Hooloovoo shared a combined existence and only occasionally split off to form an individual entity. (It could be the Great Intelligence was a member of this species.)
On whichever Akhet world they evolved, the Hooloovoo probably used their great intelligence to force others with actual forms to the manual labors they could not themselves perform - like manufacturing even the slightest things they needed.
Perhaps it's a willing symbiotic relationship which they share with the humanoids on their shared world.
But in any case, it's likely there were times whena Hooloovoo needed a more human form themselves. And so like the inhabitants of the Blessed Isle in the ancient tome "True Histories", they probably would inhabit "puppet" shells to give them that form.