i think that you get the level 48 pet from your level 48 spell quest (in your case, frost giant) im not sure though. i hope that this helped
Morgrim Darksword Magus (level 30) Necromancer
this is how ya get your level 48 pet, when ya reach level 48 your teacher will call ya over with a tutorial tip, afterwords go to your main teachers like greyrose for ice or falmea for fire, your main teacher will send you on a quest to learn your level 48 spell and when ya learn that spell ya will get your level 48 pet
note i am only level 40 ice but i know how the quest works because i saw it on youtube
At the time, I did not know my backpack was full. So I did not get it. If only there was a way I could get it now.
Aaron Winterriver Level 49 Master Thaumturge
When your backpack is full items get sent to your bank, so check there for it. (Also, if you try to use a second chance chest when your backpack is full you will receive gold instead of a treasure item.)