I have been seeing them, using them, and I am just wondering what they mean. MF_ is documented as Material Function, which clearly does not relate to animation. Knowledgeable people, can you tell me?
This would still be good to know 
For anyone who searches this in the future, I think its Mannequin_Male and Mannequin_Female
In Third Person template, MM has SkeletalMesh of Quinn. Quinn is the female mannequin.
There are mistakes in files names. This creates confusion. Please fix it in future versions of engine.
In the animation context, MF_ and MM_ are generally naming conventions rather than Unreal Engine asset types.
If you’re seeing them in animation/locomotion assets, MM_ usually refers to Motion Matching, while MF_ can be used by a particular project or Epic sample as an internal prefix for a specific animation-related asset/function. The important distinction is that Unreal doesn’t universally enforce these prefixes—the exact meaning can depend on the project, sample, or naming convention used by the author.
So I wouldn’t assume MF_ = Material Function just because that’s what it means elsewhere. The safest way to identify it is to check the asset’s actual class/type in the Content Browser or inspect where the asset is referenced.
If you can post the full asset names containing MF_ and MM_, that would make it much easier to identify exactly what the prefixes mean in that particular project.