Okay, so there are two ways we can go about that! Acata memories, or Spellbind. Since the dreamscapes are so malleable one could say Hunk's own memories of the Yellow Lion could manifest to be his Lion body if he were to be in memories Shiro was in lion shape.

It could also be that way because of Shiro's ties/friendship/bonds with him. It depends on what memories you want to explore more.