Actually Ranma's behavior fits the beast far better than Ryoga's.  Ryoga's
reaction to Akane was at first indifference(last part of Manga Volume 1),
then love (begginning of Manga Volume 2). Ranma's behavior however 
parallel the Beast's far more closely.  His initial dislike of Akane,
saving her from Ryoga's attacks (it in manga volume 1 where Ryoga's
atacking him and she gets caught in the crossfire), his growing affection
for her...  All of these match the Beast.  Ryoga might match the Beast in
form (those fangs) but Ranma matches the Beast in spirit.

I guess you like the Ryoga/Akane match huh?

Actually, no, I don't.  I like the Ranma/Akane match best.  Its just
that I thought Ryouga would make a better beast that Ranma.  I mean,
Ranma may be quite rude and all, but I don't see him as much as a beast
than Ryoga.  That's all.
-- 
 Magic Knight Kyone
--
"Do not meddle in the affairs of wizards, for they are subtle and
 quick to anger." --J.R.R. Tolkien
"Do not meddle in the affairs of dragons, for you are crunchy and
 good with ketchup." --Anonymous