Is it necessary to buy WLC to manage FAP?

Basically there are 3 different series.

FortiAP - Can be managed by FortiGate or by FortiCloud; Cannot be managed by WLC

FortiAP-U - This is the universal series, can be managed by FortiGate, FortiCloud, or WLC

Legacy Meru or 822/832 - Only managed by WLC (These are mostly being phased out as is the WLC)

Of the current products, none are solely managed by the WLC.