One of the OSR projects on my backburner requires alien sentients and flora/fauna for at least 4 planets. It's a pulpish game, think ERB's Mars and similar works from other authors but not limited to a single planet.

No magic properties for any of the entries.

What are the best candidates you can think to convert into the denizens, fauna/flora of those planets?

Anything really if put a little twist on it.
And of course a few D&D monsters are taken from pulps and scifi stories.

Most notable being the Displacer Beast, being inspired by the Cheurl from the Black Destroyer segment of Voyage of the Space Beagle.
Bemusingly some of the monsters and player races in Spelljammer are conversions of Star Frontiers aliens.

Someone many a year ago did a pretty good conversion of Gnolls into a space faring psionic race for Spelljammer based on a sci-fi novel, Witch World believe.

Some of the Alternity aliens got converted to Spellhammer too. Might not be too hard to convert the monsters from that to D&D. Same for Amazing Engine, just harder in that case. Or convert anything from Gamma World as they make great aliens too.

In my own SF setting Ghouls are a jackal-like space faring race that feed on dead flesh, but cant stand the smell and taste of murdered flesh. Making them actually well suited for forensic, pathology and detective professions.

One thing to remember about a lot of planetary romance, including Barsoom, is that they don't put a lot of work into things like ecology or coming up with realistic creatures. Instead, they tend to be Earthly analogues of something, but with a few signature traits that make them weird.

Banths are basically lions, except with extra legs, protruding eyes, and no hair. Thoats are horses, with extra legs, and heads that split in half. Calots are dogs, with extra legs, no hair, and look like frogs. These aren't necessarily close analogues. They tend to be bigger, faster, more powerful, more dangerous, more aggressive -- everything amped up. And then they have the alien overlaid on top.

So start with a theme. Barsoom's crazy about extra legs and a lack of hair. Do something else. Maybe give everything feathers, or quills, or tiny scales that cut, or they're all covered with shaggy hair. Maybe all creatures have trilateral symmetry, or huge forelimbs and relatively small hindlimbs. Maybe they have bony crests that unfold. Maybe they all have a bioluminescent lantern, that extends from their head. Maybe they have tails like lizards, that snap off. Steal ideas from real world creatures that are weird and strange or from exotic places, like lizards or amphibians, octopodes, jellyfish, insects, deep sea fish, cave fauna, black smoker creatures, axolotls, crabs, or Anomalocaris.

Pick say two of these traits, and then apply them to every creature from that planet. You're not trying to make them unique at this stage. You're trying to create a recognizable, identifiable pattern. Because if every creature is alien and weird and unique, they'll all blur together. Players will have a hard time keep them separate, or grouping them together, or remembering what planet they're from. But if you give them a few consistent lynchpins for each planet, they'll be able to distinguish the flora and fauna of each planet from the flora and fauna of the others. Each planet will then become more alive, more unique. Then give each creature a weird or extreme feature or two, that's unique to that specific creature, and call it quits. The longer the list of random weirdness, the harder it is to keep straight. So add just enough.

Start from the animal, dire animal, dinosaur, and vermin lists. Don't be afraid to boost them, or borrow monster equivalents that are tougher, or reskin. Also don't be afraid to switch up which animals you use for inspiration on different planets, for similar roles. The giant elephant-equivalents might be based on sauropods on one planet, and squid the next. The apex predators might be based on tigers on one world, and on fire ants on the next. And so on.

Orcs in Space, and each planet's underworld is ruled by Alien Drow!!!

I'd start with Pat's suggestions but an important part of adding the traits is "what makes this world unique from the other three?"

Presuming one is an Earth analogue, you've got three other places you need to make distinct and just 2-3 traits will do it without them feeling too generic.

Mars/Barsoom would have been fairly generic if it had just had low gravity and/or arid environments, but throw in being resource starved due to the planet's age and it's much more distinct.

Krypton had its ancient red sun, high gravity and tectonic activity to set it apart.

Choosing the planets' unique attributes will go a long way towards suggesting which couple of consistent features you can throw onto the life there (ex. low gravity would lend itself towards larger and thinner creatures since gravity isn't weighing them down as much; a tidally locked world is going to have three main biomes (eternal day, eternal night and eternal twilight) that can result in three variants of each species (use twilight as the default then adapt for heat/light and cold/dark variants).

Giant spiders - just give them the ability to talk, change the appearance somewhat (though they can still be a spider analogue), keep the web ability, which they use to weave cloth and rope for trade as well as for its traditional use.

Bullywugs. Can stay primitive and planet-bound, more emphasis on spears and hide clothing.

I took the standard D&D races and used them in a Sci-Fi context. They were all genetically engineered species of humans. 

Elves were made for space travel.  They had great dex scores so they could be oilots and function in zero-G environments.  They had agreater longevity so they could handle slower-than-light space travel, with the aid of cold sleep, they could live for millenia, they had low light vision for unlit areas of starships and dwrk olaces in between systems.

Dwarves were bred for High-g mining operations.  They had high con and could withstand physical hardships, disease, radiation, and poisons better than most humanoids.  They have a knack for tinkering and making things, as well as for repair.  Low light vision helped them see in the mines.  Dispite their tolerance for high gravity, many dwarven colonies functioned as asteroid miners as well, turing huge asteroids into space-faring worldships, and factories.  A dwarf colony could core an iron asteroid and spin it to achieve a more comfortable gravity in which to work. 

Orcs are genetically engineerd soldiers.  Bred to fight and to die, they are quarrelsome by nature. Their strength and con are markedly greater than the average human's.  Their  lifespan is short, reflecting their utilitarian purpose.  Lowlight vision helps with night maneuvers. 

Gnomes were meant to be technicians.  They are more Intelligent than other humanoids on a erage and have a knack for fixing and inventing things.  Greater than normal constitution helps them work longer hours. Like elves and dwarves, their oifespan was increased for space travel.  Unfortunately, their innate curiosity causes them to perform dangerous experiments, leading to a shorter than normal lifespan.

Halflings were an unforseen accident.  The Makers originally intended them to be agricultural colonists.  Making them small and wiry in stature to reauire less food, so that more products could be exported to high population worlds, but with the fall of the Great Civilization, and a surplus of food, the halflings developed culinary tastes and appeties that far outweigh their diminutive frames. 

I feel like multi armed space gorrilas are an auto include for planetary romance.

Mind flayers.

Borrowing from Larry Niven's Protector books, Kobolds are merely immature dragons. 

Dragons are created when a Mobold of exceptional ability survives several centuries as a leader to it's people.  Undergoing the transformation, it feels compelled to seclude itself and a hard shell much like a coccoon or an egg forms around its body.  After the metamorphosis occurs, the shell cracks open and a young dragon emerges. 

Psychically aware, the dragon has learned speech through absorbing the thoughts of its kobold minions.  It is also very hungry and not too picky about how it meets the nutritional needs of it's new body.  If the Kobold tribe does not have protein rich food available, they will often be consumed in a ravenous feeding frenzy, until the dragon's hunger is sated. 

If there are any kobolds left after the first few hours of the dragon's emergence, they will enthusiastically serve the dragon, recognizing it as their natural leader.  The dragon will guide the new community to a greater destiny, as the dragon encourages development of technology, and the ability to grow the tribe (and feed itself). 

Dragons and kobolds may or may not be hostile to strangers, but however they deal with adventuring starfarers, they will always put their own interests first.

How about a world with giant insects that inhabit mountain-sized hives, or a water-world with breathable water and dolphin-like tribes.