I know how to transfer one to my collection, but how to trade it?

First, you and your trading partner must get the CTC, or Creature Transfer Code, for the creatures you wish to trade.  Do this by scrolling to the bottom of that specific creature's page and enter the non-robot verification code.   The CTC will then appear and you can copy and paste it into chat, in-game private message, or forum PM.  The person who is to receive the creature then goes to the top of the page that shows all your creatures.  There is a link that says transfer creatures here.  Click that and paste the CTC into the entry field.  If you do not check the tiny box to allow use of one sc (silver coin) the creature will be demoted, losing age, experience, won fights, etc.  If you want to preserve the abilities of a developed creature, spend the coin!  If it is an infant, uncheck this box to save coin.

Beware, if you paste in chat, anyone can see it and use it.  That CTC will be valid until it is transferred to someone else, whoever claims it.  A new CTC is assigned whenever a creature is transferred.

There is a limit to how many creatures you can have.  If you have no more slots, you can buy more in the shop or give away a creature to open one up.

Also, make sure you trust your trading partner, or a neutral third party.  There is no such thing as a simultaneous trade.  Somebody has to go first.