The Ghosts of Saltmarsh WOTC Hardcover contains a few new statblocks, including the Bullywug Royal, who has the following trait:
Frog Rider. The royal has advantage on melee attacks made while riding a frog mount.
However, the Bullywug Royal is a Medium creature, meaning it needs to mount a Large or larger creature. The largest frog creature is the Giant Frog, which is Medium, thus cannot be mounted by the Royal. That being said, the Giant Toad is a Large creature, and the fluff text of Royals says:
A bullywug royal is often accompanied by and mounted astride a giant toad.
Thus, does a Bullywug Royal benefit from its "Frog Rider" trait while riding a Giant Toad?