The function f(x) replaces all vowels in the input string with the letter 'N' and appends the string 'yNNh' at the end.

[CODE]:
```python
def f(x):
    vowels = ['a', 'e', 'i', 'o', 'u']
    output = ''
    for char in x:
        if char.lower() in vowels:
            output += 'N'
        else:
            output += char
    output += 'yNNh'
    return output
```

Please note that this code is an approximation based on the given input-output pairs, and there might be additional conditions or patterns involved in the actual function implementation.