Jenny Slate and Patton Oswalt were considered for the roles of Rawhide and Mayor Rhubarb, respectively. Test animations of the characters featuring Slate's and Oswalt's voices can be found on YouTube.
Dom Kreep, the composer for the pilot, provided the screams for the two ghosts that break out of the skulls in the saloon scene.
During the scene where Rawhide crosses off Mako's wanted poster, you can see various "Wanted criminals" on the other posters, including the 4 members of the Pizza Party Podcast: Pan, Nolan, Izzi and Jim, a couple characters from Kickstarter backers, as well as Erin Fitzgerald.