Xeme
W, X, y and Z are the last four letters of the alphabet.
The answer to the riddle is "Mailbox." It starts with the letter 'M,' ends with 'X,' and can contain an endless number of letters (as in mail) inside it.
x-ray!
xerox
Xenops is a tropical rainforest bird. It begins with the letter X.
Xantus' Hummingbird and a Xenops is a bird genus
There is a bird called Terek Sandpiper which is also known as Xenus cinereus.
Flocks
Absolutely no idea. Sorry.
ibex
Twix
SUSSEX