While held, acts as one of the three items needed for Guppy.
The effect is lost if Kid's Drawing is dropped.
This includes instantly losing the transformation if dropping the trinket results in having only 2 out of 3 Guppy items required for the transformation.
Notes
If multiple copies of the trinket are obtained, they will each count as a Guppy item.
Unlike Guppy items, this trinket doesn't have the "Guppy" tag (in items_metadata.xml) and seems to be hard-coded to count toward the total. This may have to do with how trinkets in general, with or without an applicable tag, don't count toward transformations or unlock requirements.
Bugs
Bug!
Although the trinket is supposed to count as 2 Guppy items with Mom's Box according to internal item descriptions, it still counts only as one.
Guppy.
Golden Trinket: Counts as holding 2 Guppy items.
Mom's Box: No effect, even with 
The Internal Item Description claims otherwise. See bugs.
Bug!
Pet Items 
