Congratulations !!!
To put the deer to the ground just press "ALT + R", you will force the model reload that, realizing that it has been called "vehicle", will lower (by gravity) until it meets a solid obstacle.
I allowed myself to retouch your materials and the "mtl" file, for a better aesthetic rendering .. and I retouched the script ...
It's a small joke ... but the goal would be noble ...
By examining the script you should be able to understand (and reproduce) how you can define the bones of a model as a "joint", and command their movements with "rotate_joint" regardless of the existence of a pre-built animation.
https://drive.google.com/file/d/14TdGDI-QnrkKXG8MtBYhQB6vLINBz96C/view?usp=sharingThe decompression of the ".otx" file automatically creates a folder "... pakages / pat / DEER_ANIM" in the folder you have set up for the Outerra data.
You will also find the original "js" file renamed "DEER_ANIM.js.BAK", which you can restore when you get tired of my ...
Congratulations again,
best wishes for your next progress !!!
P.S.
It only occurred to me, just now, another thing ...
Probably it is sufficient to equip the deer with a mass to make it lay on the ground without the need for the "reload" ...
it's good to try ...