Do you or did you bought 1 from the Treasure of the Sage? .. To use that item, you must open the Element Nature Hexagon in the Summon Cultivation Interface of the Kyukusho Deer that has the note "New Summon Feature" .. Once that Element Nature Hexagon is opened, you will see a Cultivation Hexagon inside that has the note saying "Activate Summoning". Once you clicked that, it will show a confirmation if you are willing to summon the said summon in exchange of the needed or required items ..

But what does it do?

It summons the Deer as a companion.

You have to cultivate it until you see a hexagon with summon contracts to activate. Then use summon contract to activate it thus making the deer follow you :P

So is just a dedicated summon contract :)) i think it would have been better to be the normal one,unless that deer is a special one that can only be aquired with that contract from treasure