If a companion command does nothing or the model doesn't appear, the cause is almost always a file-placement or load-order problem. Work through these checks in order.
Check the files
1. Confirm all three plugins are installed and enabled: ModelEngine, MythicMobs and MCPets. The pack needs all of them.
2. Open each plugin's directory and make sure the matching files from the pack's Plugin Files folder are actually there.
3. Watch for accidental extra nesting, such as a folder dragged inside another folder of the same name. Files that end up one level too deep often won't be found.
4. Make sure you didn't put the MythicMobs files in the ModelEngine folder, or the other way round.
Reload in the right order
1. Run /meg reload first so the model blueprints load.
2. Then run /mm reload so MythicMobs registers the companions against those models.
3. If reloading doesn't help, do a full server restart. A restart loads everything fresh.
4. Watch the server console during the reload or restart. Error lines that mention the companion files will point you to the folder that's wrong.
Double-check the spawn command
Use the exact spawn commands from the pack's installation guide. A typo in a companion's name is an easy mistake. Run the commands with an account that has permission to use them.