Tottoko Hamtarou OVA 3: Hamuchanzu to Niji no Kuni no Oujisama - Sekai de Ichiban no Takaramono

Based on the third Hamtaro game (fourth in Japan), Prince Bo falls from the sky when the rainbow disappears. Now Hamtaro and the Ham-Hams must help him bring back the rainbow.

(Source: Wikipedia)