Give a Human entity the Alpha/Classic Walking Animation?

If you're talking about the steve-looking mobs that flailed their arms around, i don't think that's possible yet as an animation.

You would go in and edit the model (If you are using a preset, then use blockbench to make a new model for use) and make the .yRot and .xRot for the arms the same.

tho for the .xRot you would add 90 so the arm goes from facing down to forming a tpose.

Here is a working model.

//Zombie.java partial port from Classic c0.0.16a_02

public class Modelhumanoid<T extends Entity> extends EntityModel<T> {
	// This layer location should be baked with EntityRendererProvider.Context in
	// the entity renderer and passed into this model's constructor
	public static final ModelLayerLocation LAYER_LOCATION = new ModelLayerLocation(
			new ResourceLocation("modid", "humanoid"), "main");
	private final ModelPart Head;
	private final ModelPart Torso;
	private final ModelPart RightLeg;
	private final ModelPart LeftLeg;
	private final ModelPart RightArm;
	private final ModelPart LeftArm;

	public Modelhumanoid(ModelPart root) {
		this.Head = root.getChild("Head");
		this.Torso = root.getChild("Torso");
		this.RightLeg = root.getChild("RightLeg");
		this.LeftLeg = root.getChild("LeftLeg");
		this.RightArm = root.getChild("RightArm");
		this.LeftArm = root.getChild("LeftArm");
	}

	public static LayerDefinition createBodyLayer() {
		MeshDefinition meshdefinition = new MeshDefinition();
		PartDefinition partdefinition = meshdefinition.getRoot();

		PartDefinition Head = partdefinition.addOrReplaceChild("Head", CubeListBuilder.create().texOffs(0, 0).addBox(
				-4.0F, -8.0F, -4.0F, 8.0F, 8.0F, 8.0F, new CubeDeformation(0.0F)), PartPose.offset(0.0F, 0.0F, 0.0F));

		PartDefinition Torso = partdefinition.addOrReplaceChild("Torso", CubeListBuilder.create().texOffs(16, 16)
				.addBox(-4.0F, -12.0F, -2.0F, 8.0F, 12.0F, 4.0F, new CubeDeformation(0.0F)),
				PartPose.offset(0.0F, 12.0F, 0.0F));

		PartDefinition RightLeg = partdefinition.addOrReplaceChild("RightLeg", CubeListBuilder.create().texOffs(0, 16)
				.addBox(-2.0F, 0.0F, -2.0F, 4.0F, 12.0F, 4.0F, new CubeDeformation(0.0F)),
				PartPose.offset(-2.0F, 12.0F, 0.0F));

		PartDefinition LeftLeg = partdefinition.addOrReplaceChild("LeftLeg",
				CubeListBuilder.create().texOffs(0, 16).mirror()
						.addBox(-2.0F, 0.0F, -2.0F, 4.0F, 12.0F, 4.0F, new CubeDeformation(0.0F)).mirror(false),
				PartPose.offset(2.0F, 12.0F, 0.0F));

		PartDefinition RightArm = partdefinition.addOrReplaceChild("RightArm",
				CubeListBuilder.create().texOffs(40, 16).mirror()
						.addBox(-2.0F, -2.0F, -2.0F, 4.0F, 12.0F, 4.0F, new CubeDeformation(0.0F)).mirror(false),
				PartPose.offset(-6.0F, 2.0F, 0.0F));

		PartDefinition LeftArm = partdefinition.addOrReplaceChild("LeftArm",
				CubeListBuilder.create().texOffs(40, 16).mirror()
						.addBox(-2.0F, -2.0F, -2.0F, 4.0F, 12.0F, 4.0F, new CubeDeformation(0.0F)).mirror(false),
				PartPose.offset(6.0F, 2.0F, 0.0F));

		return LayerDefinition.create(meshdefinition, 64, 32);
	}

	@Override
	public void renderToBuffer(PoseStack poseStack, VertexConsumer buffer, int packedLight, int packedOverlay,
			float red, float green, float blue, float alpha) {
		Head.render(poseStack, buffer, packedLight, packedOverlay);
		Torso.render(poseStack, buffer, packedLight, packedOverlay);
		RightLeg.render(poseStack, buffer, packedLight, packedOverlay);
		LeftLeg.render(poseStack, buffer, packedLight, packedOverlay);
		RightArm.render(poseStack, buffer, packedLight, packedOverlay);
		LeftArm.render(poseStack, buffer, packedLight, packedOverlay);
	}

	public void setupAnim(T entity, float limbSwing, float limbSwingAmount, float ageInTicks, float netHeadYaw,
			float headPitch) {
		this.LeftLeg.xRot = Mth.cos(limbSwing * 1.0F) * -1.0F * limbSwingAmount;
		
		this.RightArm.xRot = Mth.cos(limbSwing * 0.6662F + (float) Math.PI) * limbSwingAmount * 2.0F;
		this.LeftArm.xRot = Mth.cos(limbSwing * 0.6662F) * limbSwingAmount * 2.0F;
		this.RightArm.zRot = Mth.cos(limbSwing * 0.2312F + (float) Math.PI) * limbSwingAmount + 45;
		this.LeftArm.zRot = Mth.cos(limbSwing * 0.2812F) * limbSwingAmount - 45;

		this.Head.yRot = netHeadYaw / (180F / (float) Math.PI);
		this.Head.xRot = headPitch / (180F / (float) Math.PI);
		this.RightLeg.xRot = Mth.cos(limbSwing * 1.0F) * 1.0F * limbSwingAmount;
	}
}

Shoot, also add " * 1.4F" to the end of the LeftLeg.xRot and RightLeg.xRot functions, it makes the legs kick out the right distance