Don't try to ride this steed unless you've got gills too.

Foundations Jumpstart

Breaching Hippocamp

Flash (You may cast this spell any time you could cast an instant.)
When Breaching Hippocamp enters, untap another target creature you control.

Flash (You may cast this spell any time you could cast an instant.)
When this creature enters, untap another target creature you control.

Jumpstart: Historic Horizons

Flash
When Breaching Hippocamp enters the battlefield, untap another target creature you control.

Theros

Flash (You may cast this spell any time you could cast an instant.)\r
When Breaching Hippocamp enters the battlefield, untap another target creature you control.