Mokona Modoki is the name of two small creatures that appear in two CLAMP manga/anime series: XXXHOLiC and Tsubasa RESERVoir CHRoNiCLE. The white one is Soel, and it's the mascot in TRC. The black one is Larg, and it's one of several mascots in HOLiC. Mokona Modoki (or just Mokona, for short) refers to any or both characters; the previous names aren't used in the series. But I'll get to that later. Both of them are important characters and plot devices in both stories, sometimes even being the nexus between the their characters.

Do not confuse Mokona Modoki (especially white) with Mokona from Magic Knight Rayearth. Indeed, they look alike, and there's a reason for it I'll explain in the story section, but they're not alternative versions of it, like it happens with a lot of Tsubasa characters.

I mentioned before that their names are Soel and Larg. We know this fact from some short stories written by Ageha Ohkawa (CLAMP's writer and leader) about the genesis and backstory of said creatures. They appeared on their official website's album some time ago. You can't find them any longer there, but there's a storybook you can buy in Japan with a recompilation of Mokona's origins and how were they "raised" by Clow Read and Yuuko Ichihara. It's called Soel to Larg: Mokona=Modoki no Bouken. The stories are childish and/or cracktacular, but it's a good thing for precanon. You can only get it in Japanese.