On the first episode of BBC's Blue Planet II they visited a coral reef and showed some odd reef-dwelling creatures as part of their establishing shots, but they did not identify the creatures.
What is this odd horned, slug-like creature? (Sort of like a slug with legs?)
I tried a Google reverse image search and it very 'helpfully' identified it as "Documentary film". So close, yet so far.
Disclaimer: This question was split off from another question since the original was effectively two questions.