Adding NBT tags to an entity?

Is this possible? I have seen countless sources online provide examples on how to do it and even though it says the entity data was modified it fails to find the element on data lookup, both when looking for the specific path and all of the data. /data merge entity @e[nbt={Item:{id:"minecraft:carrot"}},limit=1] {Silly:1b} > Modified entity data of Carrot /data get entity @e[nbt={Item:{id:"minecraft:carrot"}},limit=1] Silly > Found no elements matching Silly I have tried it on mobs as well, with no success. Upon further testing doing the same with CustomName seems to work, are NBT tags limited to a selection of pre-existing tags? How come I see custom ones online?