In my book series, there is an underwater civilization called the Lobster Empire (basically underwater Rome inhabited by Crab and Lobster people).

Anyway, what sea creatures would work best as underwater cavalry?

Preferences:

• Can breathe underwater or at least can hold its breath for hours at a time
• Easy to feed
• Is not more dangerous to the rider than its opponents
• Fast
• Can carry decent amounts of supplies
• Is not super expensive to maintain
• Reproduces relatively quickly

In my book series, there is an underwater civilization called the Lobster Empire (basically underwater Rome inhabited by Crab and Lobster people).

Stuff to know about the Lobster Empire:

• Roman-esque culture and technology
• Are more like people than lobsters (anthropomorphized, walk on two legs, wear clothes, etc).
• The way anthropomorphic animals work in this universe is that they are essentially treated as and act like humans. They walk on two legs and are sized slightly shorter than humans (the average lobster is around 5'7"). For example, if a talking lobster eats a talking fish, that is cannibalism. If a talking lobster eats a wild or farmed lobster, for example, that is completely normal.
• The Lobster Empire is looking for two types of cavalry: heavy cavalry and lighter recon units.
• The Lobster Empire only occupies sunlit, shallow regions (1-600 feet from the surface). They have no need for land troops since the entire planet is underwater, and they have no need for deep-sea units because those areas are uninhabited wasteland.

Anyway, what sea creatures would work best as underwater cavalry?

Preferences:

• Can breathe underwater or at least can hold its breath for hours at a time
• Easy to feed
• Is not more dangerous to the rider than its opponents
• Fast
• Can carry decent amounts of supplies
• Is not super expensive to maintain
• Reproduces relatively quickly