Either my kids name them, or it's something random that I think fits. Names I've come up with are
Clementine
Ursula
Gertrude
Paco
Charlene
Willa
Sweet Feet (the ridiculously fast leghorn I couldn't catch)
And a few examples of my 6-year-olds picks:
Ugly
Rainbow
Bossy
Bella
Greedy (but this...