Hive Commander shadowvilan127:
This is a great question and one that I remember asking. Here's where the phrase "although they do NOT become beasts" come into play
Sometimes you have a mission where you play with Escalation rules, this relates to the Reserves rules. The book mentions that only infantry units are deployed, while the rest are held in reserve. So any special units such as jetbikes and monstorous creatures cannot be deployed. In addition, when a unit is classified as "beasts", they also cannot be deployed and are held in reserve; your Raveners must start off in reserve in escalation missions.
However, units with leaping get a 12" assault and say they're not classified as beasts, this means that you can field units such as hormagaunts and Tyranid Warriors in the first turn. Both hormagaunts and Tyranid Warriors are classified as infantry, but with the leaping biomorph they still can deploy as normal.
There's a page in the rulebook which lists all the unit designations (infantry, jet bike, monstorus creature, vehicle, etc.). If you own the small rulebook, its one of the last pages.
Hopefully this helps.