TFor the Bulette, while I know it has an established history in D&D, there's nothing particularly evocative about it for me. It has that "land shark" label, but it doesn't look anywhere near sleek and deadly enough for that.
Personally, I'd consider a flavour and appearance re-skinning to make the Bulette a little more like the Graboids from the Tremors movies. Maybe even extend it to giving them a multi-stage life cycle, but without slavishly copying the movies.
I'm not sure I'd borrow from the graboids for them, although I could certainly see something like that for purple worms.
I think I'd like bulettes a lot more if they focused on pulling people under. A monster that pops out of the dirt and attacks you is boring. A monster that grabs one of your party members and drags them into a rapidly collapsing tunnel? That's freaky.
My favorite use for bulettes, though, is definitely as a god/pet for goblins, worshipped from a safe distance and rewarded with treats (i.e. captives) for burrowing where the goblins want. When it starts getting nippy, give it a goblin "sacrifice" that's been poisoned so it gets sick and stops trying to eat goblins.
In my last campaign, the barbarian killed a tribe's bulette and was adopted as the new god by the eyewitnesses, causing a holy war in the tribe. I wouldn't include that in an MM, obviously, but it was darkly humorous.
With the manticore, that "dimly intelligent" part doesn't do much for them. I'd consider bringing them back to purely animal intelligence, or else leave their actual intelligence score as it is, but emphasise an attitude of low, vicious cunning that dictates their interactions with both masters and prey.
I have to admit, I've never used manticores.
Associating them with evil spellcasters helps some, but they're basically custom-made for random encounters. I'd play that up by making them highly territorial with the classic taste for human flesh. Make them strong enough that they can block trade routes and terrorize wildlands and you're probably good.
I agree on the intelligence, though. That middle range between animal and low-human is just kind of awkward.
Owlbears are desperately in need of some form of hook or special feature that brings them out of being just a weird not-animal for the DM to use when he's tired of apes or bears. Stop with the "probably" malarkey about their creation, and build something interesting upon their origin story. Like, maybe their arcane origins leave them with a craving for arcane magic, and they will favour magical beasts and arcane casters as prey, singling them out with a weak innate Detect Magic sense.
In addition to eating magical prey, they like to line their nests with any magical items they find, either upon their victims or that they happen to find within their territory. An owlbear which manages to include sufficient arcane magic in its diet and surroundings may achieve a dim level of intelligence, becoming a smarter hunter and possibly learning to communicate. Arcane casters can occasionally tame an owlbear to serve as a guardian by providing a constant supply of magically-infused food and housing, but if the supply is ever interrupted for even a single day, the owlbear will return to its untameable feral nature.
Poor owlbears. They're really just in there because tradition, but there's so little to hook onto.
I like your "no, seriously, a wizard did it" angle. Would make them really handy for rangers tracking magic users and druids/fae that don't want magic users in their forests as well.
Barring that, just include a "pureblood" variant that can turn into a giant owl. Even without any explanatory text, that makes it pretty clear what their creator was trying to do and what went wrong over the years.
Pseudodragons are your standard magical pet, and a little too idealised in that role. Give them some form of culture, goals and motivations of their own and they may start getting interesting.
Yeah, they either need information about how they act in the wild or how they're created.
It's not really backwards-compatible, but I'm tempted to make them homonculi made from a mixture of the caster's blood and dragon blood in my next campaign.
Cheers!
Kinak