Flat Mount 160mm rear brake adapter
Flaanimal and Traildonkey 3 both have industry standard flat mount brake caliper mounting points that accept a 140mm rotor if no adapter is used, both front & rear. If you wish to use a 160mm rotor you need a 160mm adapter for your caliper. SRAM rear brake calipers sometimes come with the adapter, but Shimano and other brands do not.
Material: Aluminum, with Stainless Steel bolts which run from the adapter into the caliper. Frame-to-adapter bolts should use the bolts that came with the caliper or frame.
Medium strength threadlocker (typically blue, such as Loctite 243) is necessary on adapter-to-caliper bolts.