Codename shared with another phone. flounder is also the codename of the Google Nexus 9 (Wi-Fi). Codenames are only unique per manufacturer, so before flashing, check that a build names the HTC Nexus 9, not just flounder.
Want to go back to stock? Google's official factory images let you reinstall stock Android on the HTC Nexus 9. See restoring stock firmware before you need it.
Hardware
No hardware specs for this device yet.
Support
No custom ROM yet
We don't know of a custom ROM that supports the HTC Nexus 9 yet. It may only have unofficial community builds (common for newer or MediaTek devices), or none at all.
If the bootloader can be unlocked, a generic system image may still run on it: what a GSI is and when it works.
Maintain or know an official ROM for it? Tell us.
ROM availability and links come from each project. CustomDroid indexes builds, it doesn't mirror them — always download from the official project, verify the file's hash/signature, confirm it matches your exact codename, and back up before flashing.