LaPooh was Rachel's childhood dog. In "The One With The Kips", she learns that LaPooh died after being hit by an ice cream truck and dragged for nineteen blocks. Rachel learns this in the same episode that Ross has to tell her they can't see each other any more, adding to her distress.
In "The One With The Ride-Along", when Monica decided to cross reference all her photos, LaPooh was under dogs and dead. Rachel drops a box of photos on top which Monica had yet to number. To appease Monica, she decides to make Margaritas and goes to Ross' apartment to retrieve the blender and while there she hears Emily's message to Ross.