For the Musketeer I was thinking along the lins of having a prestigious bird. The bird has to have class and some sort of style to it. I was thinking along the lines of an eagle, but also added the ferret in there because it already has that friendly loyal, yet crafty look to it which could suit a Musketeer.
I have a number of ideas for the ninja. Im thinking a small monkey like Abu from Alladdin would go well with a ninja. Ive also delved into the more mythical side of a ninja and looked at little dragons and phoenix's too. The praying mantis seems like a good choice, even more so if the ninja is evil due to its appearance.
The most important sidekick here, however, would be the cat. I chose the cat because they were actually of use to ninjas. The Ninja devised an ingenious method of telling time by using a cat's eyes. Since a cat has such sensitive eyes, they adjust frequently as the sun passes through the sky. This picture illustrates how the Ninja used this knowledge to approximate time.
Ninjas were also said to carry crickets with them in a little box, they would use the noise the crickets made to cover up any small sounds made by the ninja when moving in on their target. I didnt know this before making this map, but im going to put the cricket up for consideration now also.