Who is the little fella in the urn?

I think thats Open to Interpretation.

It does kind of look like one of Carlos Huante's Concepts for the Prequels.

But again it is likely just something that HOLDS the Four Ampoules in Place so they do-not touch each other.

There Certainly is some Mechanism that is Affected by Certain Factors that Activates the Internal Components of the URNS....  something would happen that would MELT those Components and Cause the Creation of the Black Goo.

We NEVER really got a Comprehensive Explanation about the URNS and their Contents and HOW they are Activated, so we could only Speculate.

The Missing Component we dont see in the image you shared is the Slime/Goo that encases those Ampoules.

Here is a Image i made a Long While back which has the Concept Work, with other Images from the Movie and that Dubious Weyland File.

Might be a relic of the ceiling Facehugger parasite design that was developed for Jon Sphaits LV-426 script.

Although, the handle does look somewhat similar to the top of the temple on Planet 4

I was a little flippant in describing it as a 'little fella', but he definitely looks grown. Obviously the purpose is to act as a handle to lift out the vials, but it's intriguing that it has this creature-like aspect.

Certainly....

The Outer Part of the Ampoules have a like Texture, the Lower Part of the Center Piece that Connects the Ampoules likewise gas a kind of Organic Mesh looking Texture.

Interesting is so does WALTERS construction.

hox I never noticed that detail. At face value it looks a lot like a Neomorph with tentacles.

You know who it reminds me of?

Maybe they could Teach David some New Instruments to Play, or he can join them with his FLUTE!

The likeness is uncanny BigDave. Thank you for pointing out hox.

I looked on Fandom wikia and i think its a Hammerpede...