This has been a reoccurring issue for me using ArcGIS Pro - when creating a fishnet and specifying the extent from an existing layer and subsequently trying to set the cell size, I am getting a mysterious unit.
I cannot specify the unit in the parameters or the environment and it is not linked to the CRS I am using. Comparing with different measurements it seems to be in meters, which doesn't make much sense given that my set CRS is WGS84. This happens for every dataset I tried so far.
Is this a bug or am I just not seeing the problem here?