There are monsters everywhere in runescape, and you can engage them in combat. Monsters help you level up your combat and also drop some pretty good items. Certain monsters can drop things like dragon amour and weapons, rune items, and other rare and expensive items. Profitable items are usually dropped by very high level monsters, but some lower level monsters can drop some pretty good stuff too. Some good low level monsters to train on are moss giants (lv42), ogre (lv53), shadow warrior(lv48), but you need to finish Legends Quest to be able to access them), chaos druid (lv14) fire giants (lv86), ankous (lvs 75,82, and 86), and dark wizards (lv7 and lv20). These monsters drop a lot of herbs, runes, big bones, high level farming seeds, dragon spears, dragon mediums, rune scimitars, and dragon half shields (shadow warriors drop dragon half shields more often than a lot of other monsters). Obviously, there are other monsters that drop good items that are low level but these are just some good suggestions. Remember, always use caution when fighting monsters and bring some food and perhaps teleport runes to get you out of a messy situation.
High level monsters are really hard to kill unless you have a high combat level and some good food. Although they are very hard to kill, they also drop very good items. This usually consists of dragon items, or special items like abyssal whips. You should always know about the monster and what attacks it likes to use, what its weak against and what are the best items it can drop ( to make sure its worth your while). Also, it is highly recommended that you bring a team to kill any of these monsters who are in multi-combat areas. I have make a chart below, listing all the very hard monsters (with info on what they drop, attacks they use, weaknesses and other information).
King Black Dragon (lv276)- Ice, poison, dragonfire and melee attacks. Multi-combat, best drops are dragon visage, dragon medium helmet.
Kalphite queen (lv333)- Range,magic, and melee attacks. Multi-combat, best drops are dragon chainbody, dragon 2h sword. She has 2 forms, the first she had protect from range/magic on so use melee. The second form has Protect from melee on so use range/magic OR use veracs armour.
Chaos Elemental (lv305)- Teleports you across area, Un-equips all your items with a red beam attack and used a magic/range based attack. Multi-combat, best drops are dragon 2h sword, dragon medium helmet, dragon shield half. This monster is located around level 52 wild and has a HUGE attack range.
Dagannoth Kings(Rex, Prime, and Supreme- all lv303)- Magic (Prime), melee (Rex), and range (Supreme). Multi-combat, best drops are dragon woodcutting axe, bezerker ring, mud battlestaff.
Mithril Dragons (lv304)- Magic, ranged, melee, and firebreath attacks. NOT MULTI COMBAT, best drops are dragon full helmet,dragon visage.
Steel Dragons (lv246)- Melee and firebreath attacks. NON MULTI COMBAT, best drops are dragon visage, dragon legs/plateskirt, dragon shield half.
Thats about it for the best high level monsters to kill, but whenever new monsters are released that drop a new item you can make loads of money ( I'm talking LOADS) by getting the new best items they drop.
Also, while your killing monsters you may come across an item called a "treasure trail". These items show maps, puzzles, instructions or other things on how to go and find the treasure that the clue is located to. Beware of enemys like saradomin/zamorak mages while doing the clues you get from very powerful monsters (lv3 clues), they can be dangerous if not prepared. You can only have one clue scroll at a time by the way, so try to only do clues you get from good monsters (for better treasure). Maybe you will be lucky and get a high level clue scroll that leads you to VERY VALUABLE treasure, so keep your fingers crossed.