I would not suggest a monster, I would suggest Soul Wars.
try going in the stronghold of security 4th floor, you should find monsters around your level. Bring food and armor as well.
If you are not a defence pure I would suggest any monster you usually train on if you are try monks or cave bugs.
Like I said to another question, depends your level now and depends are you a member or not.
Lester demons are good and black demons. the both give good drops.
Personally, I EV train around level 45. You should give your Pokemon a Power Belt (frontier) to help train for defense and a Power Band for Special Defense. If you have neither, you can get a Macho Brace. For Defense training, go into any cave with lots of Geodudes. And for Special Defense the Old Chateau holds many Gastly that are good as well.
i know you have most likely heard this before, but i would go with hill giants, look it up how to get there, good luck.
Non-member - Cockroach Soldiers. Member - Level 25 Experiments (400xp Per kill!) You will come across these during the Creature of Fenkenstrain quest.
It was a good defense because it let Sam Houston have time to train a bigger army for the Battle of San Jacinto.
They are moderately good to train on for lower level players, yes.
train your pocket monster really good and think about it.
You can try killing rats in Lumbridge swamp if you can't afford food and armor but if you can it's a good idea to train at Barbarian Village between Varrock and Falador. The Barbarians have low defense but a fair amount of hp.
blue ribbon pigs, got to level 30 in a flash, it is in the right of nautilus, the pirates place you know? anyway keep on going right