Well, we know killer whales can definitely toss heavy seals a long ways (https://www.youtube.com/watch?v=G7WGIH35JBE), but that's a bit out of the scope of the question. The creatures are on land, and if they can carry 1000 kg, they are big.
Could a natural gunpowder or equivalent be made? The bombardier beetle is one example of something that can generate two chemicals, and mix them on demand. From Wikipedia, "The spray is produced from a reaction between two chemical compounds, hydroquinone and hydrogen peroxide, which are stored in two reservoirs in the beetle's abdomen." Our comrades over at Biology tell us the maximum pressure inside BTW:
It's kind of low, so you'd get those elements separately (or from two different creatures, which is safer and easier). If you could find other creatures that had this capability, you could milk them in some fashion, so that brings the biological bit forward.
For the cannon, you need to have something that can handle the stress of combustion or whatever pressure is driving this. In theory, if you had a cored out tree trunk that was massive, you might be able to handle that sort of pressure.
What is the pressure? Firearms folks know a bit:
They assume average pressure is about 1/4 of the peak pressure. For an iron projectile (assuming a sphere), 7.8x that of water, that's about a 3" diameter shot. From the equation that the gun folks use, peak pressure of 3000 psi, with a 24" barrel gets you into the 500 m/s range.
Bamboo might be used (ala Captain Kirk in episode with the Gorn), but Mythbusters has already busted that idea, but that's for a single bit of bamboo. Looking at mechanical properties of bamboo, tensile strength is 2000 kN/cm^2, vs 35000 kN/cm^2 for steel, about 1/15th.
If you make a piece of wood thick enough, you might be able to handle the stresses of whatever your propellant is.
For large enough thicknesses of wood, you might be able to pull this off. Got to do some additional math to check this out.
If the cannon is to be fed food, and produce the required reaction, then some sort of dual creature setup might work. One creature (or creatures) on one side of the cannon excretes compound A; another secretes compound B. When mixed, they ignite, or they become far more volatile. The problem with this is that biologically, you would be asking a creature to excrete something with high caloric/energy value, unless this can be justified for another evolutionary reason.
As far as ignition goes, this discussion talks about some possibilities:
Can hydrochloric acid and digestive enzymes cause a fire?
For full completeness, Larry Niven's stage trees should be mentioned, and various similar systems:
Could plants spread their seed to other planets?