Enhanced client, right?
The devs made a change in the last patch that caused this issue.
there's a way to remedy it by copying part of the unpatched client into a custom skin, that a dev mentioned in a reply on u.hall (see the section "Developer Feed", and you might be able to find it easier, in response to one of the posts on the patch contents).
They said they'd do something to resolve this next patch, but I can't remember what.
You can also load the game in the Classic client / 2D to rename the pets, but unlike the EC, you can't add new spaces into the name (at least, I think you can add spaces in EC - I know you could in the defunct KR client).
You might be able to get some further help in the Enhanced client forum, below this one.
