Quoting someone from a Skype group I'm in:
I did more research. One of two things is going to happen. Either you only have a single rideable Rathalos that you've bonded with because it was a runt individual that was picked on by other monsters. Or, the more likely scenario is that you are able to ride a large variety of monsters via egg gathering missions, where you collect the eggs from the wild, raise up the babies in captivity, and they increase in size over time via level-up. As such, the Rathalos in the video was a young one and still has some aging to do because it's a juvenile. Either way, I'm pretty sure you start off with Rathalos and it gets stronger as it grows. I'm 90% sure it's the latter though because Capcom even said that the plot of the game is going around trying to make better relationships with monsters, suggesting you're not just bonding with the Rathalos.EDIT: In celebration of MH4U selling over 1 million copies overseas, we're getting a free Monster Hunter 3DS HOME Menu Theme in May!