well, first make all the monsters, and then open up Half-Life model viewer, (try a search on
http://www.google.ca) and open pak0.pak then choose the model you want to see the animations with, take a look at the anims, and then using a scripted sequence in the game, make them attack the certain location. to get the creature to walk to a place,
this is EASY AS PIE, place the scripted_sequence where you want him to walk to, then in the properties, choose move_to_position-Walk or Run.