"X" is for "Xeemu"

Xeemu

No. Enc.:  1d4 (1d6)
Alignment:  Chaotic
Movement:  180' (60')
Armor Class:  7
Hit Dice:  5
Attacks:  2 or 1 (1 peck, 1 kick, or 1 weapon)
Damage:  1d6 / 1d10, or by weapon
Save:  L5
Morale:  9
Hoard Class:  VII
XP:  950

The man-sized, 400 lb xeemus are mean-spirited, aggressively stupid avians that accost any humanoids they encounter.  They love shiny trinkets and weapons, and rob travelers unlucky enough to enter their territory.

Quite dexterous, a xeemu can wield 1 weapon (via prehensile feet) in combat. However, the unique resonances of energy-based melee weapons (vibro-swords, warp-field maces, etc.) drive them into a panic.

Xeemus produce live young, and carry their hatchlings in abdominal pouches.

Mutations:  Aberrant Form (Pouch, Prehensile Feet), Increased Balance, Neural Telekinesis, Phobia (Energy HTH Weaponry) [D], Teleport

"L" is for "Leapsteed"

Leapsteed

No. Enc.:  0 (2d6)
Alignment:  Neutral
Movement:  240' (80')
Armor Class:  3 / 6
Hit Dice:  5+2
Attacks:  2 or 1 (2 kicks, or 1 headbutt)
Damage:  (1d10 / 1d10, or 2d6)
Save:  L3
Morale:  9
Hoard Class:  None
XP:  860

Leapsteeds are bison-sized amphibians adapted to drier terrestrial climes.  Thick armored plating covers their heads and upper-bodies [AC 3 when attacked from the front and sides], while their backs, abdomens, and extremities are slick and smooth [AC 6 from behind].  They travel in family herds.

Leapsteeds are omnivores, dining on vegetation, tubers, and small animals.  They also, bizarrely enough, feed on psychic energies, which they procure by mentally manipulating (via Empathy) sentient beings into physical contact.  Upon a successful Mental Attack (as if from a WIL score of 2d6+5), a leapsteed motivates the target into using it as a trusted mount.  Sticky secretions adhere the target in place on the leapsteed's back (even during full-bore movement), and the leapstead is "steered" with its rein-like antennae, which also serve as telepathic conduits.  Leapsteeds drain 1d3 HP per day from their riders.

Despite the seemingly negative implications of mind-control-based relationships, leapsteeds genuinely care for their "masters", and crave positive symbiotic bonds.  In fact, most leapsteeds find that, with enough time, they don't have to use their mutations on their riders, thanks to engendered trust and goodwill.

Leapsteeds are powerful enough to carry two riders (sitting one in front of the other) and all their gear, plus an additional 200 lbs.

Mutations:  Empathy (Modified), Neural Telepathy (Modified)

"T" is for "Tapiroid" [Metamorphosis Alpha Version]

Tapiroid

Number Usually Appearing:  1-3

Armor Class:  6

Movement (Yards / Turn):  12

Hit Dice:  6

Mutations:  Heightened Vision, Hemophilia, Illusion Generation, Mental Paralysis, Molecular Disruption, Repulsion Field Generation

Tapiroid (tapir):  This 5' tall, nocturnal biped travels in small family bands.  Being hemophiliacs, they are shy and secretive, and have evolved powerful mental mutations to avoid and deter conflict.

Tapiroids prefer heavily wooded environs, and often travel along lake and river bottoms to mask their travels.  Their prominent snouts can further elongate another 1-2' to serve as (rather clumsy) extra limbs, or makeshift snorkels.