I use to play on a server, Paradise Lost that was fairly balanced though not as well as Whispering Pines.
I've noticed that most of the end game pets are mounts. Unicorns, reptalons,Cu sidhe's ect. I personally prefer to use large and menacing pets that you would not dare to mount.
My idea is taken from that server and it is a 4 slot follower with the appearence of the Ancient Wyrm. Very rare spawn, requires 120 taming and lore just to attempt. and requires 110 peacemaking to calm it. The pets abilities are that of the White Wyrm with the exception of that it has the mele damage of the Cu sidhe minus the bleed effect.
No real name comes to mind nor the color it should be.
It is not meant to be an over powering pet, simply a pet that loves to eat gold and meat alike. and is very scary looking.