You can add any creature stat block to your character sheet in the Extras section!
It's easy to add creatures to your sheet that puts all of the detailed DDB monster information at your fingertips, in addition to updating the monster stat block information automatically when game rules would affect them. This is how you can track mounts, familiars, animal companions, wildshape forms, and other "pets".
If you click the Manage Extras button, you can see your list of Current Extras, and you can also choose to Add an Extra. The creature Categories are shown in the image below: