They are, usually! But they can transform into other Pokemon and use all of their moves and everything!

[Jimmy turns away from his Gear for a moment, crouching slightly with his hand on one knee so he can talk to the fake Magcargo.]

Hey Bubblegum, change back to normal for a sec!

[The Pokemon looks up, beady eyes blinking slowly before its body begins to shift and seems to abruptly turn into the aforementioned pink goo, shrinking until there's nothing left but a blob on the ground with a smiling face.]