Allright, Now I have no clue how to do this myself so I can't give you the advice on how to just some suggestions. First off you could ask the creator of the Game that you've seen this in how he/she did it. Second you could mess around with some sprites and try and add a weapon there and make common events to change so that the sprite is different with every other weapon they equip: So if a player equips a Short Sword to a character, you change the sprite to a Short Sword Sprite that you've created, or if they equip a Magical Wand you could make a sprite with that magical wand and make it that characters weapon in battle.
Let me know if you find anything out or if this was helpful in the least bit. I hope you find what you need, even though this topic seems to have already died...