Rhyl model
The rhyl, convincingly as it might have seemed at the time, was not a real-world animal, but rather an imaginary CGI model. As one of the two major effects houses charged with providing the production with the digital visual effects, it was Blue Sky/VIFX, responsible for the planet-bound VFX for the feature, that was to construct the CGI model of the rhyl.  read, "EXT. VILLAGE - DAY - (OPTICAL) Artim is playing with a tiny 'palm-pet', a colorful cross between a caterpillar and a jellyfish that crawls over his hand and between his fingers, as Sojef moves over with Picard and Data...", and as "palm-pet" it would remain known for the modelers at Blue Sky.

Design 

First-off a general design of the creature had to be come up with as Animation Director Mark Baldo recalled, "We gathered our full team of modelers, designers and animators, then began submitting designs based on a vague script description suggesting a cross between a caterpillar and a jellyfish. We went through some amazingly cool designs, eventually tailoring it to look like a baby seal or walrus mixed with small rodent elements. Shaun Cusick sculpted both the maquette and the 3-D computer model, which was modeled in Alias [remark: the name of the company that owns and markets the  software] and animated in Softimage. He incorporated something of my baby daughter Miranda, as well, which gave the creature these huge cute cheeks." (Cinefex, issue 77, pp. 81, 83) The producers were late in signing-off on the decision to have a palm-pet included, and Baldo elaborated on another occasion,
"That was extremely quick. They needed us on set with a maquette two weeks after they gave us the go-ahead. What was so exciting was that there was nothing, not even a cocktail-napkin sketch, to start with, although the script described the creature as a cross between a caterpillar and a jellyfish. Some of the early designs were pretty wild. Everyone who wanted to submitted a design, and it came down to a core group of our modelers and animators, including Mike DeFeo, Sean Cusick, Jim Bresnahan and myself, all drawing and going crazy trying to get ideas. Sean Cusick's sketches ended up being closest to what they had in mind, so that design was selected and refined. We then had our artists start sculpting. The first unit was shooting the scene that introduces the Palm-Pet during our first day on the set". (American Cinematographer, issue January 1999, p. 45)
Continuing, Balbo elaborated, "We opened it up to all the designers at our company to get ideas. We submitted about a dozen different designs to Jonathan and Rick, which got narrowed down to one, tailored to their specifications." Nevertheless, getting there was an arduous process as Digital Effects Supervisor Mitch Kopelman recalled, "Some of the original sketches were quite absurd. And they were all very humorous.", not few of them resembling a little seal at one point, to which Baldo added, "(...)with a big head, a big neck, a tapered body and four little flippers." It was left to Senior Animator Jim Breshnahan to actually produce a variety of concept illustrations.

Yet, thanks to Jim Hourihan's particle generating software, by then incorporated as a module in the Maya CGI software, the CGI software package of choice for Blue Sky, a plush felt was soon applied to the naked "seal". (The Secrets of Star Trek: Insurrection, p. 151)

Study and CGI models 

Yet, that being said, form and texture was only part of the big problem of getting the rhyl to life, or as Kopelman has put it, "The biggest question, since there are no actual palm-pets to base it on, was "How does this thing move?"" Part of the solution was to have three-dimensional study models built, for the CGI modelers to mull over. Those models were built by Modeler/Animator Cusick (The Secrets of Star Trek: Insurrection, pp. 151-153), the definitive one being presented to Director Jonathan Frakes, fondly remembered by Balbo, when he "(...)showed Jonathan Frakes the maquette — I'll never forget this as long as I live — he held it in his hands and said, 'What's not to love?". (American Cinematographer, January 1999, p. 45)

In the meantime, Balbo and Animator Doug Dooley, watched countless hours of tapes of caterpillars, seals and an assortment of rodents, just to determine a concept of movement of "(...)how this little creature moves, how it reacts, and what it does. We developed a background for how it behaves.", as Baldo elaborated. Consequently, Modeler Shaun Cusick constructed a CGI wire-frame model of the palm-pet, for Dooley to manipulate to see if the model was up to the required tasks. Cusick then, "(...)made a little sculpt of it, which we cast in resin so it could travel. We even put a little fur on it.", Baldo reiterated.

It was this sculpture, to be handled by Michael Welch, playing Artim, and his counterpart, Brian Scheu, playing Artim's friend, that provided the raw footage, or photographing reference plates, for Blue Sky/VIFX (East) to build their finalized CGI model upon, and as it also conveyed the very tasking CGI composition (at the time) of the palm-pet transitioning from one hand to another.

As to the CGI transition composition, Baldo has stated, "The first shot – which was supposed to be simple, with one kid passing it to another&ndas;wound up being staged up in a tree. There was a moving camera with rack focus going on through this dappled-sunlight scene, so it became a monster tracking job later. We were tracking the hands of these kid actors as well as the movement of the camera, which included a tilt and a dolly-out. Jonathan Frakes gave me so much freedom it was shocking. He said, "You're the animation director, so get up on the tree and tell the boys what you need."" (Cinefex, issue 77, p. 83) Baldo, whose filming team was further comprised of digital effects supervisors Mitch Kopelman and Christopher Scollard, added further, "Jonathan graciously allowed me to work with the boys, to explain what the little CGI palm-pet would be doing and to show them what to do with their hands. I told Michael Welch, who plays Artim, to make sure that his hand drops down a little bit when the other boy hands him the animal. That will help the animators, because it gives them a physical cue as to when the full weight of the creature is in the actor's hand." It subsequently took further advanced manipulation of the footage to produce the final footage as featured, including footage shot with the use of two small physical objects, a plastic cube and a light gray ball, for additional post-production film editing purposes. (The Secrets of Star Trek: Insurrection, pp. 153-155) In that respect Kopelman commented,

<blockquote>"We use mathematics to figure out the field of view for our computer camera. Since the cube is a very simple perspective object, we can use it to confirm that our mathematics are correct. We'll create a computerized cube thew same dimensions as the physical cub the actor is holding, line them up, and make sure that any lens issues are resolved perfectly. And we use the gray ball as a lighting reference. On such a simple geometric shape it's very easy to get all the directional information regarding the lighting environment. We'll place a computer generated sphere in the same space as the real sphere and play with the lighting until it is pretty well matched on each. When the computerized sphere looks just like the reference photo, we know we're almost there.(...)The biggest chore is the rotoscoping. We created a CG hand in rough animation, and then we had to match each move of the animated hand in the computer to the real hand. It takes a lot of time."</blockquote>
Chiming in, Baldo added, "We also shot the resin model for reference. It told us the scale, how the light hits it and how light interacts with the fur, which is different from the skin. So the model wasn't only a tool to help actors visualize, it was also a tool for us back at the studio.(...)We take all our cues from the live footage. We even have to know what film stock Matt Leonetti uses because we have to match the film grain in our CGI world." (The Secrets of Star Trek: Insurrection, pp. 154-155)

Originally, the modelers at Blue Sky, animated the rhyl as being a fast moving critter, much like real world rodents. Yet the producers rethought the characteristics of the palm-pet and decided to considerably slow down its movements. This meant that Blue Sky had to redo the animation at a slower pace and that the animation already done, had to be discarded. (Cinefex, issue 77, p. 83)

The enormous amount of work entailed at the time notwithstanding, Visual Effects Supervisor Jim Rygiel did state somewhat proudly, "I think they pulled it off pretty well. It's something you've never seen, but it had to have the fine detail of a living, breathing thing. They managed to make it look cute, as opposed to looking like a slug." (American Cinematographer'', January 1999, p. 46)

Studio models