I'm a huge fan of reusing flannel pieces whenever possible. No need to reinvent the wheel and all that. Half of my sets have notes in them saying that one piece or another is filed with another set.
For my flannelization of Deborah Bruss' book Book! Book! Book! I reused the pig, cow and duck from my I Had A Rooster set.
Then I added a blue horse (just in case I ever decide to do anything with Eric Carle's The Artist Who Painted a Blue Horse. You always want to be thinking ahead.)
An adorable goat
And finally, the frog who says "Read it! Read it!" instead of "Ribbit, Ribbit"
I use the flannel pieces in conjunction with the book since the book by itself is a little long for a preschool crowd. Before I start I ask for help and pass out all the flannel pieces to kids in the crowd. When their animal shows up for the first time I ask them to come put it up on the board and tell me what sound their animal makes. Then the story of the animals who want something to do and the librarian who only understands people-talk really begins.