How can i make a ridable mob?

There is no such tag. Your best option is to put an invisible horse there and teleport the visible mob to the horse, but even that only works if you can actually click it, which might not work with a ravager, because it's so big.

You can ride an invisible horse that is riding the entity that you want to ride. You would appear to be floating a bit above that mob, but it does work.

To make a rideable zombie you can use this command:

/summon minecraft:zombie ~ ~ ~ {Passengers:[{id:"minecraft:horse",Tame:1,ActiveEffects:[{Id:14,Amplifier:0,Duration:2000000000,ShowParticles:0b}]}]}

To see what it looks like if the horse is not invisible you can use this command, or you can wait until the duration of the invisibility runs out after about 3 years:

/summon minecraft:zombie ~ ~ ~ {Passengers:[{id:"minecraft:horse",Tame:1}]}

This command summons a rideable ender dragon, but it will be very hard to sit on it, because the horse will be much higher than you´d expect and the dragon moves a lot.

/summon minecraft:ender_dragon ~ ~ ~ {Passengers:[{id:"minecraft:horse",Tame:1,ActiveEffects:[{Id:14,Amplifier:0,Duration:200000000,ShowParticles:0b}]}]}

Note: To ride the mob that you want to ride, you need to right click the invisible horse, NOT the mob that you try to ride

Rideable ravager command : /summon minecraft:ravager ~ ~ ~ {Passengers:[{id:"minecraft:horse",Tame:1,ActiveEffects:[{Id:14,Amplifier:0,Duration:2000000000,ShowParticles:0b}]}]}