Just like many other DMs out there in the worlds, I see some monsters in the books and don't like them. either that or I like them partly but I don't like some key things about them which mostly ruin the whole monster for me.
At first, when it came to monsters I partly liked and partly didn't, I would try to house-rule the monster to be different. That just ended up confusing everyone, including myself. Say you have three or four Players sitting there and you mention a Vampire, if they have read the MM or had any previous experience with one, they have a fixed view of the monster in mind already. Then there I go, trying to re-define that monster on them. It doesn't work.
What to do? I still don't like how the monster works per the book. Then I decided, why not just create my own and give it a different name instead. This way, when I introduce the monster by the different name, there is no per-conceived idea to fight against.
One example I can offer is the Vampire, but you never saw that coming, did ya? I hate level drain. I think the mechanic of tying life force to experience levels is completely nonsensical. Also, I dislike treating the Vampire like a large rodent or snake where it will only get a single attack and it uses it's most unique attack, the blood sucking bite, every time. These are intelligent humanoid monsters, to give them such a simple attack makes no sense.
Many of the other features and abilities of the MM Vampire I can accept. After failing miserably with the house-ruled change I decided to make my own. I called it a "Strigoi", the old Gypsy name for a creature like this. It shares most of the basics of the Vampire from the book. But, when it comes to attacks, I gave it something different.
First off, I gave it 3 attacks instead d of one. Why? Well because first of all, this kind of creature, mythologically speaking, is not going to fight like a typical human anyway. Only after it has tried magical based or other attacks will this monster go hand to hand with an opponent. They aren't likely to use a weapon like a sword or such.
Only if pressed into a corner will it fight tooth and nail to get out. So I figured IF it goes into melee combat with a "hand to hand" attack, it will attack animal like with a claw/claw/bite and the bite is just as any normal creature biting, it's not trying to do a complex blood slurping bite, it is just tearing with it's teeth.
Beyond that, I gave it a special attack as a "Draining Bite". This attack also drains the life force from it's victim at 2 points per round if the monster can hold the attack from round to round. There is a chance, a slight one, for a victim to break free, but at this point, the victim is in a swoon or paralyzed state and pretty much just can't resist enough to move away. (Save vs Paralyzation -4)
Instead of draining 2 levels from experience, it drains points from the victims Constitution score. Should it drain all points, the victim is dead and will rise later as a minor Strigoi. However, if somehow the Strigoi has drained at least half of the PC's Con points and the "Drain Bite" attack is broken, the PC will likely live but is "infected" and upon dying at a later time, will rise afterward as a minor Strigoi.
There are more specifics and details but I gave you the gist of it. I have what I wanted, a vampire that makes sense to me ans is just as badass if not a tad bit moreso (in my own opinion) than the one in the book.
I've done this with a few monsters and I don't even use the the original monsters from the books any more. I just use my own instead. However, if I'm running a module (which I rarely if ever do anymore) that has BtB vampire in it, and Players who are not regulars in my game, then I will usually leave it alone and play it BtB, disliking every instance of it.