Adapt 3. (If this creature has no +1/+1 counters on it, put three +1/+1 counters on it.)
Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
Now I'm curious about the mechanical differences between the two abilities.
I can think of one difference: Monstrosity has more flexible prerequisites than Adapt: A non-monstrous creature could have other counters on it already (from a different source) and still become monstrous, while a non-adapted creature couldn't become adapted if it already has a counter on it.
Are there more mechanical differences between Adapt and Monstrosity?