Maybe through source code never tried it.
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Yes ofc, you can add it, just check the pets on the bmd files on the client and on the server side the txt files for it, add the right item, than you can put into the "pet" slot.
Without epic coding you can make 3 type of pet, 1 like angels, satan, 1 like panda, rudolf etc..., and one under the character like fenrir, horse, unicorn, dinorant.
Just copy the right code and the animation will be the same, but it will works.