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

ハムちゃんずと虹の国の王子さま 〜せかいでいちばんのたからもの〜

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)