Monster Knight is a cancelled third person action-rpg developed by Insomniac that was supposed to be released for PS2. The game was never officially announced, but its design document , created in 1999 and which contained some concept art that we can see below, was shown to IGN in august 2012.
From the few info available about it, Monster knight was probably a mix between Zelda and Pokemon: in fact, the player main’s objective was to explore a large world and capture the various creatures that he encountered.
However, in this game monsters could be acquired also by negotiation or after pursuing and catching them, and they were used by the player not as fighters but as equipment or transport. Every creature had a particular class, growth pattern and personality.
For more information check IGN article .