my leashed common Day Zentu doesn't show up on Kumos forums. Just an empty image. When I hover over the empty image, there's a red "Failed to load pet data" text that appears.

The image it's trying to load is https://img.subeta.net/pets/zentu_day which has this error:

404 Not Found

Code: NoSuchKey

Message: The specified key does not exist.

Key: pets/zentu_day

RequestId: 54TSKSY1NVBD7TVN

HostId: HRsf6oxYKo3CABrOqHaWIG8nd83S18CzS3RHPPbKfN31KUtBKBBvA+mCDWU7+c+81ZSP8E0ogYM=

An Error Occurred While Attempting to Retrieve a Custom Error Document

Code: NoSuchKey

Message: The specified key does not exist.

Key: error.html

Spoiler contains image with red error text to show you how it looks - image doesn't show cursor hovering over the missing image, but that's what was happening.

SPOILER (click to toggle)