To basically yes. As long as it is technically below your CR rating (maybe 17-20 maybe higher if you use epic levels)
The only thing I would say must be the case. That it must exist.
- CR 13 Vampire with Mage Level (as long as it's below your rating, this is your punishment)
- Level 17 – 20 Fighter (Fine)
- Mindflayer Warlock (probably not for two reasons, 1 if you turn into a MINDFLAYER that will fool you, but the second part is impossible without a patron in hand)
- Adult Dragon with Paladin Level? (Carry on it sounds terrifying)
The following, however, would at least not be dismissed out of hand.
- A completely new creature that never existed (you need to be able to understand it)
- Some perfect CR rating legal and GM excepted (yet no. Pitchforks down people let me finish)
Basically, if you want to invent a completely new creature, you should let them do that … IC do it that is. After all, we got the owl bear and a few other creatures. Let them go through the process of creating this new creature as a world first. After that, they can turn into True Polymorph.
Could you just fiat GM?
Of course you could, but tell me, what's more fun when the wizard does some earthshaking things and creates a whole new creature that populates the world. Travel to places that are far in the wind to find the essence needed for manufacturing. A touch of heaven, a hint of wind elemental, and for a good measure, throw a fox in it. depending on how complex they want to do it. The wizard and the group could do all these things before lunch (Planeshift + Teleport) or until the end of the week. Also, it could give the group some more ideas to add to the creature. Damn, the rest of the party could join in as well.
Seriously, although it's the 9th level magic, you have to accept the OP at this point.