Bunnygeist spawns in the Shower, Tsughos and Angello spawns in the Dungeons. They all spawn in the final Biome, which you only visit after all other biomes have been cleared forthat loop/round (10 floors).
...Also, it is possible to go to C:\Users\<YOURUSERNAMEHERE>\AppData\Local\Ghosts_and_Japes\resources\progress, go to each of the prg files, and enter...
bunnygeist angello tsughos
...and so forth for all the ghosts you want. I wonder how CMCDEV feels about people suggesting doing the same unlock everything shitck you see from THAT genre. Maybe they'll make that sort of game and ruin my aspirations :(
There's also blackouts and the party hat, which spawns ghosts constantly. I think the spirit horn might spawn Tsughos, but I didn't test it.