Both characters are obsessed with a certain legendary creature and spend their free time trying to prove that such creatures exist.

Why are certain creatures attractive to some of us, and some not?

Thus, I knew from the start that a certain creature would be trouble.

Upon arriving in Geneva, he sees the monster near the site of the murder, and becomes certain the creature is the killer.

Many hazards, including spikes, water, lava, and certain creatures can kill the ball.

Then there are certain living creatures, including humans, who conceal such a focus within themselves.

Because it was not there, you punished certain creatures for its lack.

Some spells require material components, which can be obtained by killing certain creatures.

Divine power which inspired fear and awe was thought to be manifested in certain creatures.

The book also explains how certain creatures of the mythology that were slain by heroes are, apparently, still alive.