The use of the phrase “terrible lizards” in the flavor text clues that each picture represents a dinosaur. In fact, each picture is a drawing of the translation of the Greek genus name.
arm lizard = BRACHIOSAURUS
horse dragon = HIPPODRACO
three-horn face = TRICERATOPS
thunder lizard = BRONTOSAURUS
heavy claw = BARYONYX
plate lizard = STEGOSAURUS*
Taking the boxed letters from each answer and placing them in the blanks at the bottom of the page gives the phrase BIRD ROBBER, and a quick internet search yields the translation, and the final answer, ORNITHOLESTES. This is probably the most difficult puzzle to have appeared on this website to date. Well done to everybody who figured it out.
*(The literal translation of stegosaurus is actually “roofed lizard”, though “plated lizard” is also a commonly given epithet. Apologies if this confused any solvers.)