Platypus is a MOOSE-based application created to enable custom MOOSE problems to be set up and solved on both GPU and CPU architectures using the MFEM finite element library. It is based off of the MOOSE electromagnetics app Apollo that uses MFEM for solving electromagnetics problems in a variety of formulations.
Platypus functionality has now been merged upstream into MOOSE, and development of the MFEM backend for MOOSE has now migrated there.