At the risk of defending a comic book movie, many stories of old (especially about battles) were told and retold over many years/generations. Exaggerations are introduced and grow over time. To someone who has never seen an elephant and is describing one that was described to him that was described to him, etc, it would not be uncommon to call it a 50ft beast with razor-sharp tusks, etc.
As you can tell by the end of
300, most of the movie is actually being told by one-eye-guy, while rallying the troops for full-on war some time after the events at Thermopylae . You probably don't rally too many troops by saying, "
Yeah, they had some animals I didn't really recognize, but since they are from a foreign land, that's perfectly understandable and they weren't really anything special." No, you say, "
Holy FUCK guys - they got demons and sorcerers and unicorns with crocodiles for cocks! But we're Spartans, and just 3 of us caught and tamed the 4-ton, 12-ft tall lions and rode them into battle eating Persians on the left and and ass-fucking and then eating the ones on the right. Sparta! FUCK YEAH!"
Also, it was a movie, not a History Channel special.