Code (Text):

1. command /pets:
2.     permission: server.owner
3.     trigger:
4.         pets(player)
5.        
6.        
7.  
8.        
9. on inventory click:
10.     if name of player's current inventory contains "&aPets":
11.         cancel event
12.         if clicked inventory != player's inventory:
13.             if index of event-slot = 49:
14.                 close player's inventory
15.             if index of event-slot = 51:
16.                 delete {current::pet::%player's uuid%}
17.                 send "&cYou Removed your pet!"
18.                
19.        
20. function pets(P: player):
21.     open chest inventory with 6 rows named "&aPets" to {_P}
22.     set slot integers between 45 and 54 of {_P}'s current inventory to black stained glass pane named ""
23.     set slot 49 of {_P}'s current inventory to barrier named "&c&lClose"
24.     set slot 51 of {_P}'s current inventory to tnt named "&cRemove Pet"
25.    
26.     set {_var} to -1
27.     loop  {pets::%{_P}'s uuid%}:
28.     add 1 to {_var}
29.     set slot {_var} of {_P}'s current inventory to loop-value