Maksim, I began to make note of Mongoose Traveller's alien description system and Traveller5's system, but I think he's specifically going CT-Only on this one.
Given that there were three published levels of detail for aliens in CT (Short LibDat, Contact article, Module) with no real shorthand appearing, you are probably best served by studying the Contact articles for examples of brief but informative prose.
I think Jim is right, here. These three forms are complementary and give us a pretty good guide as to what needs describing.
And I think, in particular, if you set all of the Contact! articles side-by-side, you'd have not only a good idea of the guide, but hints for random tables as a "spur to the imagination".
If I were to go the CT route, I would presume a Universal Alien Profile (UAP) with six to nine elements to it. It could contain a size digit overloaded with atmosphere code; a characteristic "leaning" digit indicating which physical characteristic is stronger and which is weaker; a structural digit (symmetry plus limb groups); a manipulator digit; and three social/cultural digits. Up to five two-letter codes might detail commonly found variations. Additional text would itemize unusual features.
Off the cuff:
Code:
Droyne 42-H00K Ps Wi S-1 E-1
Aslan 82-P00S Ag Ca S+1 E+1
Vargr 62-H00C S-1 D+1
Hiver 85-G00? Cu S-1 E-1
K'Kree 84-H00K Ag He S+1 E+1
1. SIZE/AIR (2D):
2 - Std (~72kg), Thin Atmo
3 - Small (~50kg), Thin Atmo
4 - Small (~50kg), Std Atmo
5 - Small, Tainted Atmo
6 - Std (~72kg), Std Atmo
7 - Std, Tainted Atmo
8 - Large (~100kg+), Std Atmo
9 - Large, Tainted Atmo
A - Std, Exotic Atmo
B - Large, Exotic Atmo
C - Small, Exotic Atmo
2. STRUCTURE (2D-2):
0 - Radial, multiple limbs/groups
1 - Asymmetric, odd limb groups
2 - Bilateral, two limb groups
3 - Trilateral, three limb groups
4 - Bilateral, more than two limb groups
5 - 5-arm radial
6 - Trilateral, more than two limb groups
7 - 7-arm radial
8 - Bilateral horizontal
9 - Trilateral horizontal
A - Asymmetric Basker/Collector
===DASH===
3. MANIPULATOR:
G - Graspers
H - Hands
P - Dextrous Paws
T - Tentacles
etc
4. SOCIAL-1:
??
5. SOCIAL-2:
??
6. SOCIAL-3:
? - Unclassifiable
C - Charismatic
K - Casted
R - Robotic (indicates a Robot "species")
S - Sociocratic?
more?
COMMON CODES:
Ag: Aggressive
Ca: Carnivorous (strict)
Cu: Curious
D+/-N: Dexterity + or -
E+/-N: Endurance + or -
He: Herbivorous (strict)
Lk: Lucky
Ps: Particularly psionic
S+/-N: Strength + or -
Wi: Winged