Suii is a type of meme within the yukkuri fandom involving yukkuris riding a pink cart, which makes a "Suiiii-" sound. The "Suiiii-" sound is a onomatopoeia for moving smoothly, like "Whooosh" in English. It is usually treated as the yukkuri equivalent of a "car".
According to some stories, a Suii is merely a makeshift cart, a plank with four wheels employed by yukkuris to move at speeds beyond their usual locomotion ways. It's unknown how a yukkuri manages to steer, stop or even accelerate its Suii. It has been humorously implied that some Suii may have a joystick device lodged where the anyaru is,
However, according to other stories, like anko214, a Suii is somewhat a living being, much like an U-Pack, thus able to sense the wishes and the orders of a yukkuri and move along, even growing up from a koyukkuri-sized baby Suii to an adult sized Suii (even a Dosu-sized). In this intepretation some evolved and well cared for Suiis are able to "heal" from damage by themselves, possibly because of the Belief Power their yukkuri owner hold, and also return to their rightful owner if stolen or abandoned, despite still obeying to the orders of every other yukkuri happens to ride them.
A Suii may be used by a single yukkuri, or an entire group of them. Especially koyukkuris, being able to fit on an adult-sized Suii, may ride all together as a form of common play activity.
Another humorous takes on the Suii meme involve a parent yukkuri teaching a koyukkuri to ride a Suii, much like a human parent would teach a human child to ride a bicycle, or adult yukkuris carring a "Suii License", acting as a Driver's License for Suii driving.
Within Yukkuri FandomEdit
Much like U-Packs, Suiiis are becoming more and more left out in yukkuri fandom, which is going for a more "real" direction (cities and Japan compared to forests and Gensokyo). Supports of the Suiiii claim that the much more violent (and popular) side of the yukkuri fandom has brought the downfall of the Suiii and U-Pack. Still, the Suiiis have been able to make their way into the "real" fandom, though simply by taking the form of a cart that yukkuri can ride on.
In some stories taking place in the "real world", pet shops are shown selling carts called "Suiiis" to yukkuri owner. In these stories, the Suiiis act something akin to a hamster ball.