Blender to Unity Animation Deformations

My animations are deformed in Unity, mostly on the legs of the model, but you can also see it on the head:

I found it particulary weird that the mesh deforms this way in the head, there's no bone in blender that I can move to make the mesh look like this:



What I tried:
- Import blend file directly to Unity
- Export to fbx from Blender, then import to Unity (I followed 4 possible configurations from different answers, nothing changed)
- Checked that the weight paint of vertices was ok
- Removed some bones from the legs, it helped but the mesh still deforms in different ways

What can I do to fix this situation so that the rig+mesh animation matches what I'm seeing on Blender?

 

I have the same issue with this

 

Sorry for not very useful input but Is it a bug or it really is a horse siren ?

 

It's a horse siren. If you check the video I posted, the weirdest behaviour is on the animation of the legs. Still to this day I haven't been able to get this to work with the armature, and used a baked frame-by-frame animation.