This is one of those things I lowkey knew, but it sounds so unnatural I tend to pretend I don't know it and add the s.
As witnessed by my website, which uses the correct name...
http://www.etherella.com/scrapbook2/merry_treats.htm...except as you can see in the URL and the link in the link frame to her page
I can confirm the UK card also calls her Merry Treat.
The same is true for Nilly (Vanilla Treat), hence why I called her Nilly as a kid >.>
And I always see people using the spelling "Tuneful". Which is only correct if you're in the UK
The US card gives her name a double ll...