They are offering this cat:
Welcome to the trader! This site will help you and a friend trade cats with one another from your savefiles.
Start by inputting the username with which other users may find you:
Please select your save file. For me, it is at the path:
c:/Users/[username]/AppData/Roaming/Glaiel Games/Mewgenics/76561198021052905/saves/steamcampaign01.sav
It's probably a good idea to make a copy/backup, just in case :)