There really is no definitive answer for this question because there are very few details about the battle itself. What's known is that Inu no Taishou battled Ryuukotsusei in his dog form but was unable to kill him, so he sealed him away using one of his fangs or claws (I can't remember which), receiving fatal injuries either before or during that process.
While this purely speculation, it may be possible that Inu no Taishou felt more comfortable/confident fighting in his true form, or thought that his true form would be more effective against such a large foe. It may also be this decision that cost him his life. Again: this is purely speculation. Damaijin (talk) 02:15, March 25, 2013 (UTC)