The companion type is not allowed validation error.
 

I'm trying out a new character concept, of a druid who rides a giant mantis. I added a level of barbarian with the mounted fury archetype after 7 levels of druid, and I get a validation error; Mantis, Giant: The companion type is not allowed Cavaliers. Allowed for Druids, Vermin Companion.

Am i missing something as to why this is not allowed?

Thanks.

If you have more than one class that grants you an animal companion/mount, you'll only get one companion, so in order for the effective druid levels of the two classes to stack, you need to pick a companion that's a legal choice for both of your classes, not just one of them, and the Mantis is only a legal choice for your druid, not your mounted fury (who uses the Cavalier list).

Thanks Mathias. Can you quote me the reference that mounted fury uses the cavalier list please.

Well, "Bestial Mount" only allows you to pick either Camel or Horse (which are the same choices allowed with the Cavalier's "Mount" ability).

Mounted fury archetype

At 5th level, the mounted fury gains the service of a feral mount. This ability functions as a druid’s animal companion, using the barbarian’s level –4 as her effective druid level. This companion must be one that she is capable of riding and is suitable as a mount. A Medium barbarian can select a camel or a horse. A Small barbarian can select a pony or a wolf, but can also select a boar or a dog if she is at least 8th level. Whenever a barbarian is raging while mounted on her bestial mount, the mount gains a +2 morale bonus to its Strength.