If you have a pet on your trade list and it gets traded away, it will leave behind a glitched slot on your trade list until you reload. The glitch slot will appear with the tags of the item to the right of it, and if you remove it, it takes that item off trade. Then if you close your inventory and open it again, the glitched slot moves back one space, and the item that was left of it before gets moved to the right of it. The glitched slot has the tags of the new item.
If the glitched slot keeps getting removed, eventually everything on your trade list will get removed. The only way to get rid of it is to remove the item to the right of it. When you do that, the glitched slot gets converted into the item you just removed when you open your inventory again, and the pesky slot will be gone.
If more than one pet was traded away, any of the glitched slots to the right of the first one can be removed without anything happening. If only the first one gets removed, the item to the left of it goes to the right end of the glitched slot chain when the inventory is reopened. The item that was to the right of the glitched slots before gets removed from the trade list, and all the glitched slots adopt the tags of the new item.
Reloading gets rid of all glitched pet slots that are on your trade list. You can only see these glitched slots on your own trade list. If glitched slots have the tags of a masterpiece, clicking the view button on any of them just views the masterpiece it got the tags from.