Tottoko Hamtarou OVA 3: Hamuchanzu to Niji no Kuni no Oujisama - Sekai de Ichiban no Takaramono
Genres: Adventure, Comedy
Type: OVA, Episodes: 1
Aired: Mar 24, 2004
Studio: TMS Entertainment
Source: Unknown
Rating: G - All Ages
Synopsis:
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)