https://mcstacker.bimbimma.com/

/give @p command_block

Once the site has loaded, select what type of mob you want, for example: zombie, skeleton, cave spider

(Next to 'Entity:')

Basically, if you fill in these values when you spawn the mob, it will be launched a certain direction, and distance. This is only really useful for minecarts, but can be used to make some cool things such as cannons.

No Gravity: If ticked the mob will not fall if there is no blocks under it, only really useful for armour stands. Automatically added with NoAI.
FallFlying: If ticked the mob will be able to use an elytra if they have one.
PersistenceRequired: If ticked, the mob will never despawn. This is automatically added if you give the mob a name, or any items.
NoAI: If ticked, the mob's AI will be disabled, the mob cannot move, be interacted with, be attacked, attack, etc.
Silent: If ticked, the mob will not make noise. (Ambient, footsteps, damage, etc)

Rotation: Change where the mob will look when it is spawned. Only really useful for armour stands, or mobs with NoAI.

Yaw: The entity's rotation around the Y axis, due west is 0. Cannot be set higher than 360.
Pitch: The entity's declination from the horizon (pitch). Horizontal is 0, positive values look down, set to -90, 90.

Positive values: How many ticks the mob is on fire. (20 ticks = 1 second)
Negative values: How many ticks a mob can stand in fire before being set on fire. (20 ticks = 1 second)

Health: The amount of health points a mob has, cannot be more than max health. (2 health points = 1 heart)
Max Health: The max amount of health points a mob can have, for example if the mobs health is 10, and the mob has a max health of 30, if you threw a healing (or harming if we're talking about undead) potion at the mob, it would be healed, though if max health was 10 then it would not be healed.

How many blocks away a mob can see you, most mobs this defaults to 16. If you set this to 10, and stood 14 blocks away from the mob, the mob would not attack you. If you stood 8 blocks away though, it would attack you.

Chance to block knockback from attacks, 1.0 is 100%

0.1 seems to be about the speed of a player walking, 0.4 seems to be about /speed 8 (essentials command). I suggest giving the mob speed effect instead of this.

Attack Damage: The amount of damage each attack does, in half hearts.

Armour: The amount of armour the mob has. 0-30

Invulnerable: If ticked, only creative players can damage the mob, cannot be moved by players, and item frames can only be destroyed if their supports are removed.

Glowing: If ticked, the mob is given a permanent effect like spectral, this can be laggy on a few computers.

Left Handed: If ticked the mob becomes left handed.

Set the scoreboard team this mob belongs too.

The path to the loot table if the mob is killed.

Select one, set the amplifier (if [potion effect] is set to 1, then it will act like [potion effect] 2), set the duration (how long it is active, in seconds), and whether it should hide particles or not.

The name of this mob. Hiding the name no longer has any use as tags have been added.

The tags the mob should have, seperate with commas. Tags are used for easy targetting (tag=[tag])

Set a certain amount, of a certain item, with certain attributes and enchantments, and set which slot it should be in. Nearly any item will be visible if put on head.

There is a few more, entity specific tags that I am not going to say here, I've covered the ones you will need for custom mobs.