Esplin dedicated a lot of resources to using Alloran's body to acquire creatures from across space, which are rarely repeated. I've listed the ones we've seen in the first book where they appear. Chances are high that there are many more. I'm linking repeatedly to
Monster-Man 08, who made a major undertaking in designing nearly every alien in the series. I'm not really set on most of these aliens looking like his designs but it's cool to have a reference! I have also named the animals that don't have any mentioned in canon, just because it's awkward to use them otherwise.
1.
Djabala, a small six-legged climbing herbivore. (mandatory first morph, possibly seen in #38)
2.
Kafit bird, a many-winged carnivorous flier, fast but not very maneuverable, which catches and spears small prey with its beak. (#18)
3.
Taxxon, ten-foot-long centipede. They're sentient but plagued with an endless shrieking hunger for flesh that's kicked up to eleven by the smell of blood, even their own. Fragile-bodied. Comfortable on land, fast on the surface of the water, can eat tunnels in dirt faster than they could be dug with hand tools. (Andalite Chronicles)
4.
Hork-Bajir, seven-foot-tall lizard man with bladed limbs. They're sentient, tree-climbing herbivores, strong and fast, and heal rapidly from injuries. (Andalite Chronicles)
5.
Antarean Bogg, a giant monster with tree trunk legs and teeth as long as human arms. (#1)
6.
Eight-everythinged monster, a tree-tall creature with superheated blood which can create fireballs in its mouth and hurl them. Let's call it a qarrin.(#1)
7.
Vanarx, smaller than a human, adheres to a person's ear and tries to suck a brain parasite out. (#2)
8.
Stony giant, three-legged rock monster as tall as a telephone pole, with a comically small head. Let's call it a Lamlat(#2)
9.
Mardrut, whale-sized entity that swims via jet propulsion and hundreds of tiny thrashing fins. Slow, not very maneuverable, untiring. (#4)
10. Human male. Described in 33 as tall, distinguished, agile, and superficially "your average, benign suit".(#6)
11.
Lerdethak, treelike monster with a big mouth and thousands of tentacles, very adept at capturing and subduing, can slither at a good pace through jungle. (#11)
12.
Lebtin javelin fish, a big yellow stingray, fast but not maneuverable, that inflates itself with water to shoot broom-handle-sized spears underwater. (#12)
13.
Sea serpent, fifty feet long, yellow, has a big mouth but is kind of fragile. Let's call it a Hoscan Serpent(#15)
14.
Flying Porcupine, quilled winged beast that is heavily armored and slow-flying. Let's call it a spiny katcharam.(#17)
15.
Dule Fansa, a purple toothy thing that can extend its sharp-tipped arms like evil yo yos. (#20)
16. New human male, not described. (#20)
17.
Lizard thing, tall and pockmarked with a vulnerable throat. Let's call it a Surrus Monster(#21)
18.
Aria, a human female. Thin, deeply tanned, with dark hair. (#23)
19.
Kaftid, eight-legged slimy seahorse-thing that spits potent acid.
20.
Medusa head creature, loosely resembles a human head with many knife-tipped tentacles. Let's call it a Deirdaris.(#24)
21. Giant anteater. It's... it's complicated.(#24)
22.
B'heeon, five-armed monster that sprays prey with water then snaps it up with a three-foot-wide tongue. (Alternamorphs)
23.
Aquatic eyeball with eyelash tentacles that numb at a touch, like jellyfish tendrils. Let's call it a vigvig.(#29)
24.
Mountain crab, translucent climbing crustacean beast that can change colors and become difficult to see. Let's call it an Alachiral Scaler.(#30)
25.
Luminar, big chubby humanoid wreathed in fire. (#36)
26.
Putrid creature, giant heap of reeking steaming flesh impervious to a polar bear's teeth and claws. Let's call it an Ord Dagees.(#37)
27.
Acidic creature coated in chemical tar that it can fling from its short tentacles. Let's call it an inkekin.(#39)
28.
Bievilerd, a 'living stealth bomber' with many sharp teeth. (#45)
29.
Reptilian gorilla, huge and acid-sweating. Let's call it a beko-beast(#49)
30.
The Goo, red puddle of mobile Jell-O, suffocating and impervious to claws and teeth. Let's call it a gadri pool.(#49)
31.
Giant armored land squid, all spiked tentacles with a barbed tongue. Let's call it a giant mammut.(#50)
32. Another human morph, some high ranking military official, "short and tan and built like a bulldog." (#51)
33.
Octopus thing with twenty eyes and twenty arms. Let's call it a griagua.(#52)
Book 27 also has a morph described as "Some hideous creation from some far-distant planet. Huge! Deadly. We couldn't defeat all his Hork-Bajir and Taxxons. Let alone this monster" which I'm taking to be one of the aforementioned monsters.
Book 36 also has a deep-sea morph observed through echolocation and described as "a bit like a giant squid", used for escape, which likewise.
Book 52 has an unseen morph that somehow allows Visser One to escape or survive the Yeerk Pool explosion.