Animations generated by Adobe's Mixamo.com are not fully compatible with Unreal Engine 4 and 5, most notably when it comes to IK or root motion extraction. Mixamo Converter software is a mini-app that processes FBX files downloaded from Mixamo and adapts them to Unreal's conventions.
Adobe's Mixamo.com is a free online resource for humanoid animation, where users select a suitable stock character (or upload their own), and then pick from a set of 2000+ animations and poses.
The selected animation or pose is instantly adapted to the custom character (in terms of gait and foot IK), and the result can be viewed in 3D before downloading. At the previewing stage, the animation or pose can be mirrored sideways, and parameters such as "Character Arm-Space" can be tweaked for extra versatility. Users can also trim the length of an animation or adjust its speed.
Apart from the 100+ stock characters, Mixamo can apply its poses and animations to fully custom humanoids. Users can either upload a non-rigged character mesh that will be auto-rigged upon upload, or they can provide an already-rigged character, as long as it matches Mixamo's conventions.
A major inconvenience is that each Mixamo animation needs to be downloaded individually (if it isn't bundled in one of the 38 "packs"). Other notable shortcomings are: lack of hand IK, awkward bone rotations for some animations, off-frame keys, and a lack of Unreal-ready root bone motion.
Mixamo Converter cleans up the animation keys (enforcing uniform sampling at 30 frames per second) and enables full Unreal compatibility, by adding animated root and IK bones. Furthermore, sensible joint constraints are enforced, and "twist" corrections for limb sections are added where applicable.
As of 2022, the Unreal Engine has had at least three generations of humanoid rigs. The "Blue Man" a.k.a. HeroTPP was used in 4.7 and earlier, replaced by the familiar SK_Mannequin in 4.8 (June 2015). Then, in 4.26 (December 2020) a female version of SK_Mannequin was added, along with a Control Rig.
Unreal Engine 5 (April 2022) introduced the MetaHuman rig and two new mannequins, Manny (male) and Quinn (female). "Simple" versions of Manny and Quinn are similar to UE4's SK_Mannequin, with extra sections for the spine and neck, metacarpal bones, extra twist correctives at elbows and knees, and two extra auxiliary bones, "interaction" and "center of gravity". Full versions of the two UE5 mannequins have many additional corrective bones, which helps eliminating uncanny deformations even for extreme poses.
N.B. For Mixamo to correctly animate the fingers, toes and head of an Unreal character, the uploaded skeleton needs to have additional bones at the toes, fingertips and head crown. For further compatibility, the skeleton must adhere to Mixamo's bone-naming convention where applicable, and the root bone should be stripped.
You can find a set of Unreal mannequins that have been optimized for Mixamo, and can be animated in the same way as stock characters or auto-rigged ones. After the animation is generated and downloaded, it is Mixamo Converter's task to restore the missing bones (root bone, IK, etc) and to animate them correctly.
Mixamo Converter is a Windows app, it requires the Visual C++ redistributable package (14MB). Autodesk's official libfbxsdk.dll (included) is used for FBX operations.