Hallowed Fantasy features an official resource pack, which adds special textures for certain Pixelmon, such as Shadow Mewtwo. Before joining the server, download the official resource pack from this link and put it into your launcher's "Resource Packs" folder.

For further instructions on how to install the Resource Pack, check out the guide on how to install our Hallowed Music Resource Pack. It will work for this one as well (although you will need to download the Hallowed Textures Resource Pack from this page instead of the music pack). Link to the guide: http://www.hallowedfantasy.com/wiki/m/46288037/page/How_to_install_Hallowed_Music