Decent Haste, Speed equips, and Speed potions

What stacks, if anything?

Speed Potion do not stack with Decent Haste. Everything else pretty much stacks together.

Speed from equips and passive skills -> Stacks
Speed from active skills -> do not stack (with each other)

Now that we are on the subject.

Would a crossbow's speed stack with the jaguars speed? (For a WH of course)

The jaguar's speed would be a skill boost, so an equips speed would stack. Not sure if the jaguar itself is active or passive, but in this case it doesn't matter.

The jaguar's speed would be a skill boost, so an equips speed would stack. Not sure if the jaguar itself is active or passive, but in this case it doesn't matter.

Wrong. Some skills change your speed to a set amount (aka battleship, speed is set on 80% and nothing will change it - buffs or slow).
After looking at post-bb jaguar stats, I'm pretty amazed. max 1st job jaguar has set 160% base speed, max 3rd job jag skill increases that to 180%. Pretty insane imho.

Mounts set your speed/jump to a certain value. Nothing changes that value.