The first thing to understand is that Unity uses a left-handed coordinate system with the convention:
- x+ = "right"
- y+ = "up"
- z+ = "forward"
(You can see this in the naming of the static property basis vectors in the Vector3 type)
That means that Unity will expect most assets to be modelled/arranged in their prefab parent to "face" in the z+ direction.
When you're browsing assets, you usually want to be looking at the front of the asset - to see the face of a character for example, not the back of their head. That means that the Unity asset preview looks at the assets from the z+ side, meaning that it's looking back along the z axis. You can verify this by making a quick little prefab with colour-coded nubs pointing along each positive axis, then looking at how it's oriented in the preview:
So, that solves your first problem: the object is not changing its transformation between the asset preview and the scene view. You just happen to have your scene view camera placed behind the object in the z axis, while the asset preview assumes you want to look at objects from in front.
Now, what's going on with the difference between Houdini and Unity?
Houdini uses a different coordinate system. It's right-handed, which means that to convert to Unity's left-handed scheme, one position axis needs to be negated. Houdini's export to Unity chooses to negate the x axis. So something that points in the x- direction in Houdini will point in the x+ direction in Unity. This conversion step is necessary so that if you're modelling a chiral object like a hand, a left hand in Houdini should import as a left hand in Unity, rather than an inside-out right hand (which is what you'd get without the negation).
You have not shown us the transform inspector for your flipper object, but if you find it's imported with an unwanted rotation, you can tick the "Bake Axis Conversion" box in the mesh import settings inspector to apply transformation changes to the vertex data itself, rather than the rotation property.
