Kuroa Chimera (Werewolf Transformation)

A werewolf named Takaomi among the main characters in this manga. Below are raw scans, but you can read some translated chapters online.
In chapter 2 Takaomi gets into a fight with a lizard dude:

In chapter 3 the fight continues, but eventually Takaomi loses control of his beast side and transforms all the way.

In chapter 7 we're treated to a flashback of Takaomi's first transformation. Yes, he had different hair back then.

In ch. 13 we get a short reversion scene in a flashback: