I know that there's pike in the Belgrade Lakes and I assume that there is elsewhere too. They're considered invasives and not all that welcome.
There's muskie too but that fishery is limited.
Smallmouth are barely tolerated but there's strong fisheries in the Androscoggin the Penobscott, and the "down east" lakes.
Brook trout are Maine's heritage. 90% of all native brookies in the USA live in Maine. In the small streams the little guys can be very abundant. Earlier this year in a feeder stream I saw a school that had at least 300 fish in a pool about the size of a SUV.
Fly fisherman are usually more interested in the the larger brook trout and land locked salmon in both rivers and ponds.The more well known rivers can get crowded but the thousands of trout ponds see a lot less pressure.
I'm a lousy photographer, but here's a good sized wild brookie from a remote pond earlier this year
Here's a friend with a smallie from a 'downeast " lake last month
A Kennebago land locked salmon