Tried taming one today. Did not go well. It's a simple process but it can go wrong very easily. Ensure your new friend is alone. A pack of these will absolutely destroy you. Keep watch for any packs roaming near. If your shadowmane wanders into a pack, it won't leave. Recommend killing any that are new but be mindful they can knock you off your mount. That being said I had a wyvern with me which made escaping easy when I was knocked off as I only needed to wait a few seconds to mount again and fly off.

Not sure if it's a glitch but I noticed the tame meter does not go down but will reset every day. That being said you should have have plenty time to catch any extra fish without worrying about your tame meter.

Note: I play on unofficial server with custom settings so you may notice differences.