Mythic calls these trophies "pixie boards." There are eleven possible color combinations.
To make these trophies, give rainbow sprite remains to an apprentice taxidermist. (Each market has two apprentice taxidermists, but only one will accept these remains.)
In addition to rainbow sprites, which aways turn into 4-on-a-board trophies, the game includes six named mobs which turn into square trophy boards with a single pixie on them. For information on these named mobs, see Glare, Red, Melancholic Fairy Queen, Tiendora, Windchaser Naihera, and Veviel.
Many thanks to Grunnelle, Libeclair, and Neum of Gaheris and to Awf Chute of Merlin.