The subject of unusual mounts and Ride checks has been brought up before. I however have not seen anything regarding Saurion Champion's dinosaur mounts. Now according to their archtype they can pick any mount as long as it's a dinosaur. In my case I chose a Deinonychus which is essentially a larger version of raptor. The archtype loses a lot but gives you a free feat as well in this case Undersize mount.
My question is do I take the -5 Ride check because it's not really meant to be a mount.
So far as I know there isn't a list of what's considered to be a creature that is "ill suited to be a mount". So, it would be a DM call if the creature falls into that category or not. Just because it only runs around on two legs, it doesn't mean that it is ill-suited, Axe Beaks for example only have two legs but are listed specifically as something that cavalier's can ride, so they clearly aren't ill-suited to serve as a mount.
I think this rule is meant to cover scenarios like when a halfling wants to ride on the shoulders of a human.