You can put on and off a stealth ring , use an invisibility spell (utana vid), use the spell Creature Illusion (utevo res ina "creature") and wait till it wears off or use a spellwand and wait till the rat / green frog / chicken costume wears off
The ones Spaldix mentioned work.
Additionally you can go to the Incredibly Old Witch south west of Thais and she will change you into various creatures for short period which, when it wears off will revert you back to your original form
This is true of any monster that can transform you,such as Werewolves and Dark Apprentices
Costume Bags don't last forever, only 5 hours of total online time and then you'll turn back to your normal self. Also, you can use another costume bag to turn yourself into a different monster!
However, if impatient there's various ways to remove Costume Bags:
Source: https://tibia.fandom.com/wiki/Costume_Bag_(Common)
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