These are the famous Nonnef version 2 replacement feet for Combiner Wars or Unite Warriors combiners. These are 3D-printed and come on sprues, unassembled.
Here is the original product information: https://www.nonnef.com/products/combiner-wars-foot-upgrade
Here are the assembly instructions: https://www.nonnef.com/blogs/news/128760643-cw-feet-and-hand-assembly-instructions
IMPORTANT: This listing is for a pair of the BLACK feet (shown unassembled in second image).