Monster Creation Terror Bird

One of my players sent a group of ghouls wandering the desert. I rolled a Monstrous Shadow terrain encounter and got the humble Ostrich on the monster roll. Sweet! Ostriches are frightening if you think about it. So instead of merely adding damage and giving the ostrich 11 HD and boosting its damage, I decided to use the Monster Creation rules and the entry for the Giant Ostrich to create The Terror Bird.

This has raised some challenges for me. I want to be consistent with the scaling from Ostrich to Giant Ostrich. My first barrier is understanding its body form. Is the Ostrich accipitrine in body form or a hybrid?

If anybody can foresee other challenges or has further advice in scaling up the Giant Ostrich to an 11 HD Terror Bird please help!

I will probably have more questions soon. I’m wondering about its movement speed, for instance. If the Ostrich is a hybrid body form that would explain a lot. Maybe accipitrine and dinosaurian?

BTW, this forum is basically dead, you’d get far faster help on the Discord. Reverse-engineering BME from weight and HD gives BME 1.62, which is a fair bit more than accipitrine. HD 11 at that BME gives a weight of 2,028 pounds. That’s what I’d call properly Terror…