Bipedal vs quadrupedal: how many legs
This is about the number of legs the creature walks on.
Bipedal. Two legs. Humans, ostriches, T-rex, kangaroos, most fantasy humanoids.
Quadrupedal. Four legs. Dogs, horses, cows, lizards, most "beast" enemies.
Hexapodal. Six legs. Insects, spiders' weird cousins.
Octopodal. Eight legs. Spiders.
In game dev, the leg count matters because it changes how locomotion works. A bipedal walk cycle is a balance problem (the character is upright on two legs and falls over without active balance). A quadruped walk cycle is more stable but has way more legs to coordinate. Inverse kinematics (IK) for placing feet on uneven ground is harder the more legs you have.
When someone says "we need to handle this if they're bipedal," they usually mean the animation system, the balance system, or the IK foot placement is different.
Humanoid: shaped like a person
Humanoid is about overall body plan, not leg count.
A humanoid has:
Humans are humanoid. Elves, orcs, robots that look like people, aliens with two arms and two legs and a head, all humanoid.
A horse is bipedal? No, quadrupedal, AND not humanoid. A T-rex is bipedal but NOT humanoid (no arms in the human sense, weird body proportions).
Humanoid matters in game engines because they have specific tools for humanoid skeletons: motion capture libraries, animation retargeting (taking an animation made for one humanoid character and applying it to another), pre-built IK rigs. Unity's Humanoid Animation system, Unreal's Mannequin system, all assume the character has a humanoid skeleton.
If you make a non-humanoid character, you usually have to set up the rig from scratch.
Anthropomorphic: animal that acts human
A step further than humanoid. Anthropomorphic means an animal (or non-human thing) that's been redesigned with human characteristics: walking on two legs, having hands, wearing clothes, speaking.
Sonic the Hedgehog is anthropomorphic. Bugs Bunny is anthropomorphic. Most furry character designs are anthropomorphic.
The shape is humanoid (two arms, two legs, head on top). The species is animal. The behavior is human.
In games, anthropomorphic characters can usually use humanoid animation systems because their skeleton matches a human one closely enough.
Other body plans
Once you leave bipedal and quadrupedal, things get interesting.
Serpentine. Snake-like. No legs. Movement is a slithering animation, often along a curve. Implementing snake movement is a whole topic on its own.
Centauroid. Half humanoid, half quadruped. A centaur. Top half is a human torso with arms. Bottom half is a horse with four legs. Six limbs total.
Tentacular. Octopus-like. Many flexible appendages instead of rigid limbs. Animation is usually done with bone chains and physics rather than IK.
Avian. Bird-shaped. Bipedal but with wings instead of arms. Different gait, different balance, often flight to handle.
Aquatic. Fish-shaped. No legs at all. Movement is based on body undulation. Often handled with vertex shaders for the swimming animation.
Insectoid. Insect body plan. Multiple legs, segmented body, often antenna and mandibles.
