AbdulAlhazred
Legend
I usually start with the monster or villain.
For me its kind of a back and forth sort of thing, sometimes I have a vision of a monster and then its place in things comes clear (or not, in which case it may end up in my notebook as a reminder so I can use it in some future time). Sometimes I have a spot that NEEDS a monster. I think usually the concept comes first, but it could also be a situational concept.
1) 'The 1000 legged one' - this was just me feeling Cthulhuoid one week. I was thinking of the description of Wilbur Whately's brother in The Dunwich Horror, crashing through the trees and leaving weird trackways everywhere. The party was adventuring in a woods under the influence of an evil sorcerer, so out it came! This was a totally custom monster made to fit my mood.
2) 'The Sorcerers Daughter' - The 4e Lamia monster entry seemed cool, a creature made out of bugs. The Green Adept (the sorcerer) was sort of a Kyuss-like kind of guy, mostly worm-eaten, so it seemed apropos that his daughter would be made of bugs! I think I used the monster pretty much stock from the book. Then I came up with a scenario for it. I definitely started with the monster, but create the lore myself, though it wasn't totally inconsistent with the MM version.
3) 'The Owl Bear' - The party had to traverse an underground cave/river system to get to their destination, and I imagined what it would be like to have this horrible fearsome beast snatching characters one at a time from the darkness with its speed and stealth while they blundered around. I can't remember exactly how it went, but I created a situation where they were forced to put out all their lights. It wasn't pretty, though! The owl bear itself, and its lore, got this one in my head.
4) 'The Ghost Wolf King' - I had this idea to have a scene set in a sawmill with the saw about to chop the damsel in half, and also already had this theme going with a group of werewolf-like ghosts from the plane of shadow. Add one log flume for the PCs to ride in on (yeah, this scenario was very kitsch) and a White Dragon reskinned as a giant nasty ghost wolf-thing with a nasty howl (the breath weapon) and it was awesome! Definitely not inspired by ANY monster lore from 4e (I also recycled jackalweres and a couple other monsters as the footsoldiers of the monster group).
5) 'The Juggernaut' - This was an ancient dwarven mining construct, which the party managed to activate during a battle in an old mine. Its main effect was to go crashing through roof supports until it brought the whole mine down around everyone's ears. Then there was the fun running battle to get out, with the loot. I think I used some sort of ogre or something as the basis for the construct. Again, the situation was the starting point on this one.
I think what I'd say is, that monster lore can be useful, up to a point. I thought 4e MM1 had some good and bad points on that front (the lore check stuff and the encounter groups were useful, the lack of even a basic description of most monsters seemed a bit odd). MM3 was a little heavy on the lore/story side sometimes. It was not BAD when they were creating a whole new monster type, like the Banderhobbs, but other times it was not so useful.