EXP files are files for exported creatures in Creatures 1 and Creatures 2. EXP files include a snapshot of the chemical condition of the norn, the genetic D-DNA, the state of the norn brain and information about pregnancies. A common trick for preserving the original exp file in Creatures 1 was to open its properties and check 'read-only'. The game would not then delete the original file upon importing the creature, although it made the game hang a bit.
Similar to SFC files, the EXP file format is a serialized version of internal MFC objects.