The patterns that I found all started at 6mos., but I was dealing with a 3 month old, so I folded the pattern to reduce it to the baby's measurements.
It was still too big when I did the fitting, but I was in the ballpark. I thought black satin wings and ears would look good, with contrast purple stitching.
I ended up using the same basic ear pattern for my own bat costume...
Here is the body, finished...
The pattern had a center back zip opening, which seems like kind of limited access for a baby costume, so I added inseam snaps, like you might see on a long-leg onesie, and kept the back zip for dressing ease.
Totally different it challenges than I normally work with, so overall this was an interesting project.