Although Tropey cannot speak in a traditional sense, the trial takes place in a fantasy or sci-fi setting in which there are reliable methods for confirming his "testimony," making Tropey a potentially valuable source of information and therefore a valid witness.
Inverted: A human being is called to testify in a court of animals.
Subverted: When the next witness is called and comes to the stand, it is a man accompanied by a dog, presented in such a way as to make it seem like the dog is the witness and the man his handler - it's actually a blind man with a seeing-eye dog.
Double Subverted: The seeing-eye dog ends up providing critical evidence or testimony.
Averted: Nothing that isn't human is called up to testify.
Enforced: "We need to liven up this comedic series' courtroom episode with some Courtroom Antics!"
Lampshaded: "What's wrong with an animal testifying? Half the people convicted already act like animals."
Exploited: "Tropey! Tell them I'm innocent, and I'll give you... a small piece of meat!" "DEAL!"
Discussed: "Did he just call the dog to testify?!"
Conversed: "That wouldn't hold up in any court. Well, OK, maybe in Ruritania."
Deconstructed: This wouldn't hold up in any Real Life courtroom, and is usually done when the lawyer is getting desperate.
Reconstructed: The lawyer has done his homework, and only calls up material witnesses. Since Tropey is a material witness, and able to speak and reason, the lawyer is justified in calling him up to testify.
Played For Drama: Bernie's client is losing the case badly, despite being innocent; calling up Tropey is a desperate last-ditch tactic to turn the trial around.