Since there seem to be several, and the question is more or less asking for a (limited) list, let's make one! And let's make it a CW-answer to have everyone contribute.
If you find another one, make it a separate answer to get voted on, then add it to this list.
Jigglypuff
The answer given in the episode, Jigglypuff as seen from above.
Voltorb
One of the suggested answers in the episode.
Foongus
Foongus would be a trick answer like the one in the episode, since it only works when seen from above.
Ditto
A Ditto is capable of transforming into any other Pokémon, so into any round one as well.
Diglett
A Diglett looks as if it could be quite round when seen from above, although it does have a nose that may poke out.
Ghastly
A Ghastly is spherical as well, if we disregard its aura.
Sandaconda
Sandaconda, when viewed head-on, would give a spherical silhouette.
Drifloon
Drifloon, a balloon-ghost pokemon that likes to carry children away, is also a round shape, and when you remove the strings (or view from above like Jigglypuff) it could certainly match the image from the show.
Sandshrew
In the anime, a Sandshrew appears to be able to roll up into a uniform sphere, and might work as another trick answer.