Unico's mom?

First of all, if Unico's a unicorn why does he look like a kitten? Which I always wondered.

Based on vague recollections of 1983 or 1984 at most, I think that Unico and his brother or sister were lost at that they had to go to the end of the world to find their mom who was like a bigger cat and not a unicorn at all and then that they found her in the black place where she was sitting around on a rainbow. Thoughts? It must be the same movie or a similar production company or at least a similar anime produced at the same time.