So, the chicken says exactly what Scootaloo thinks on the second panel and then she's getting feel really weird, because this chicken is accompanying with her? Perfect joke! And I love Scoot's facial expressions, especially in panel 2!
In one of the episodes, the girls are trying to find a chicken that escaped from Fluttershy's coop. While clucking to try and coaz it out, Applebloom is chastised by Scootaloo who says 'That is NOT how you call a chicken!'
Applebloom then says that Scootaloo is a chicken for being scared of the forest, at which Applebloom starts to call out Scootaloo's name to coaz out the chicken as a joke.
Everyone thought the joke was funny and so Scootaloo became a chicken.
When the Cutie Mark Crusaders we're being baby sat by Fluttershy in the Episode "The Stare Master", they sneak out to find a lost Chicken of Fluttershy's. While searching the Everfree, Scootaloo complains to Applebloom that she wasn't calling the Chicken the right way. To taunt her friend, Applebloom starts saying "SCOOTALOO, SCOOT SCOOT SCOOTALOO!" instead of "here chicken chicken chicken". The Fans (me included) have since referred to Scootaloo as a Chicken.