I Was In Creative With My BestFriend, Foe And I realized That I Could Not Put Him On A Lead Because As Soon As I Clicked On him (he is a horse) it would Go Into The Pet Menu, So My Suggestion Is That When You Shift And Then Click Your Pet With The Lead It Will Go On Him As Normal. :D:D:D:D Hope You Enjoyed :D:p;)
You can't stick a lead on a pet, don't think there is a plugin for it, and it's to much drama just to add it, But why would you need a lead on a pet, you walk off it'll still follow you, or if you want it to stay still, don't move.