If the game’s main point is accuracy, then it would certainly make sense that the shape of a body part itself affects the overall motion of an animal. However, most biomechanics is very complex and would require lots of tedious calculations for the engine.
Instead, these various attributes could be mimicked but arbitrarily set for different classes of body parts based on research and simplified into parameters such as coefficients of velocity and acceleration vectors or other survival factors.
For instance, a body, such as that of a wolf, would have attributes like (out of 100 optimally, but it can go beyond 100 in theory)
mass: 50 (lower mass is better)
These attributes of a body would then get averaged with another body part such as a foot that also affects all these motions, say for instance, the foot of a swan
In this case,
speed is the maximum velocity the animal can attain on flat terrain
acceleration is how quickly the animal can attain its max speed, slow down, and change direction without slipping
balance is the percentage of acceleration an animal retains as it walks over uneven terrain or quickly changes terrain
stamina is the total capacity (or total ATP accessible by the animal’s myofibrils) that an animal can use to undergo strenuous activity such as running or swimming
swim is the percentage of speed that the animal retains in the water (which can be higher than 100)
dyanamics is the drag coefficient an animal suffers at high speeds either in water or in the air
(or possibly underground)
support is the percentage of weight the animal can carry in proportion to its size (under its own weight and of other objects) before suffering a penalty to stamina, speed, acceleration, swim, dynamics and jump.
jump is the max height that an animal can attain on flat terrain by means of its own locomotion, not including wings.
endurance is the rate that stamina decreases as a result of undergoing strenuous activity
mass is how much weight the body part contributes to the animal.
tempering is the coefficient of its other stats the animal retains in proportion to the environment’s deviation from the animal’s naturally optimal temperature.
exothermicity a percentage of the whatever metabolic benefit the animal may receive from its environment as the environment’s temperature deviates from its optimal temperature, based on exothermic animals
resistance is the percentage in reduction of damage from corrosive chemicals or toxins and also possibly damage-dealing temperatures
The final result would be
There are of course other possibilities, but this also gives players the option to specialize if they want to. They can choose to rely primarily on something like water by not adding legs and thus not forcing the animal’s stats to average with something of inferior swim or dynamics in order to make fish.
Or, they can specialize in health parts (and arms, legs, feet and hands will have a tendency to be more fragile than bodies) at the expense of speed by making an animal like a centipede that has legs but no feet, or a snake that has neither, so a player wouldn’t be forced to make higher-health bug legs or a reptile body average its high health stat with something obviously more fragile like a swan foot.