The Ankylosaurus can be found by cleaning the third area 'Lava Falls'. After the area is cleaned and the shop debt is paid off you can buy Ironwood Tree seeds from the online shop. Plant and water these in the seed plots nearby. Once they are fully grown the Ankylosaurus will appear and can be fed using the Tough Fruit from these trees. However you are unable to ride them. You will unlock the ability to buy saddles from the online shop during the fourth area 'Frosty Fields'.
After buying the correct saddle and having it delivered simply run back to the Ankylosaurus and press
to ride it.