Final answer:
In the Keeper of the Lost Cities series, Keefe's nickname for Silveny is 'Glitter Butt,' reflecting the alicorn's sparkling characteristics and the playful bond between them.
Step-by-step explanation:
The nickname Keefe uses for Silveny in the Keeper of the Lost Cities series by Shannon Messenger is “Glitter Butt”. This term of endearment reflects the character's playful relationship with the alicorn, a creature similar to a unicorn but with wings, characterized by its shimmering, glittery fur. Keefe, known for his humor and light-heartedness, affectionately uses this nickname to capture Silveny's unique and magical qualities.