Among other Pokemon, Toxicroak is an excellent Pokemon for Zoroark to impersonate. There are two main reasons for this. The first being typing advantages, and the second being overlapping movepools. Toxicroak's resistance to fighting and bug means that you'll rarely have to take Super Effective attacks as Zoroark, and the disguise will lure in Psychic types attempting to kill a Toxicroak, In the event of a Psychic type coming in, Zoroark is essentially presented with a free turn to do whatever it wants, as it's dark typing means it won't be affected by Psychic type moves. It can set up a Nasty Plot, put up a Substitute, or just go straight for the kill.
The second reason that Zoroark impersonates Toxicroak so well is that Toxicroak is capable of learning many common moves utilized by Zoroark, such as Nasty Plot, Dark Pulse, Suckerpunch, Taunt, Swords Dance, Knock Off, and Focus Blast, to name a few.
Just be careful about switching into water or fire moves, or use in weather, as people might start to get suspicious after they see you don't have Dry Skin. However, even if they get suspicious, it's likely that they won't guess that you have a Zoroark.