What are some examples of igneous rocks that are plutonic?
Granite, Diorite and Tonalite
Now, I have named felsic igneous rocks which are common in our crust, it is not so common to have mafic plutons in our crust but it does happen, for example at hotspots.
If we intruded some mafic magma into the crust it would form something along the lines of a gabbro if we had a pluton of it.
The three I mentioned above fall under the broad category of granitoids and what makes them plutonic is that they cool really slowly. We can see this by looking at grain size, the bigger the mineral, the longer it took to cool. It is just like making noodles, if you take them out of the boiling water quickly they will be quite thin, but leave them in longer and they will absorb more and become thick, same principle.
So rule of thumb for igneous rocks, small grain size = quick cooling and large grain size = slow cooling.