Not surprisingly, the scene-stealing stars of the film turn out to be the CGI sabre-tooth tiger, woolly mammoths, and especially the big, two-legged birds, whose technical name is Phorusrhacos but is more colloquially called the "terror bird." 10,000 B.C.'s scariest moments involve Phorusrhacos attacks and are clearly inspired by the Velociraptor scenes from the Jurassic Park movies.
10,000 B.C. does not, however, mark the big screen debut of the Phorusrhacos. The fierce, flightless bird made a memorable bow in 1961's Mysterious Island, which featured stop-motion effects by the legendary Ray Harryhausen. 10,000 B.C. presents more of a villainous, dramatic turn, but in Mysterious Island, the Phorusrhacos showed off more of its comedic chops, as the soundtrack music suggests:
[kml_flashembed movie="http://www.youtube.com/v/xBacIM8dUkY" width="425" height="350" wmode="transparent" /]
Speaking of 10,000 B.C., yesterday I found this interesting factoid from its "FAQ" page on the Internet Movie Database, which I reproduce verbatim:
"What year is the movie based on?"
"Some people believe the year 10,000 B.C. but IMDB actually discovered this movie was made in 2007."
It has since been taken down, alas.