You can use the Andorra Bestiary from my saga. Just rip and fudge stats as needed.
An easy way to do it from scratch is starting with the aforementioned Book of Mundane Beasts. Compare the imagined creature to a mundane beast. A manticore is like a lion, fudge stats from that base and add something reasonable for the spiky tail thing. A giant snake is a viper with a big boost to strength and size. And a lot of classic "monsters" are humanoid variants (goblins and such)
Magical powers can be complex, and to help you I thought up a super simple easy system.
Come up with a Might Score you think is fair. I propose a scale of 10 weak, 20 moderate, 30 strong, 40 powerful, and 50 boss player.
:mrgreen:
Might equals a number of extra Virtue points. Ignore complexities from books you don't have. Stick with Virtues. Retcon latter.
You can also use points to directly buy Powers instead of Virtues. Each point can buy ten levels worth of powers. Levels may be combined or divided as needed. If the power is equal to or less than the creatures might it has no activation cost. Otherwise, divide level by might score (round down) to determine activation cost. This cost is paid in points from the Might Pool (equal to score, spent as needed and replenished over the course of a day). You may decide that an individual can spend a point of Confidence or a Fatigue level instead, no matter the activation cost. Powers are designed like Hermetic spells, but they can break some of the rules or have different restrictions as you decided fits. Call this + or - a magnitude. For Experience Points, Might x 15 as an average, x10 if stupid, x20 if smart, and x30 if wise.
The rules from RoP: Magic do not work this at all. But that is a whole book, and I wrote a single paragraph that can give you a framework out of chaos you can work with until you can get that book (which is, in mu opinion, the very best sourcebook fifth edition has yet produced, though it is not perfect). It also serves as a good example of how to make up your own rules, something that you may wind up doing a lot in Ars Magica 
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