A 21-year-old man has been fined after pleading guilty to the ill-treatment of an animal when he filmed a man allegedly attacking a quokka on Rottnest Island.

Corey Douglas James filmed the man allegedly kicking the quokka and sending it flying into the wall of a concrete fence at the front yard of an accommodation unit on the holiday island in February.

The initial charge of cruelty to animals was downgraded in the Fremantle Magistrates Court on Monday, after the state prosecutor said the quokka could not be found to determine its condition after the incident.

The prosecutor also told the court James filmed the incident, which he then posted on Snapchat, and he can be heard laughing in the video while he made no attempt to stop the attack.

Corey Douglas James leaves the Fremantle Court after pleading guilty to the ill-treatment of an animal. ( ABC News: Eliza Laschon )

James' lawyer, Nicole Young, told the court her client was "extremely intoxicated" at the time of the incident and was "deeply remorseful" of the "pathetic, immature, cruel act".

Ms Young said James was a boilermaker who had arrived on the island on Saturday with a group of friends for a music festival.

She said since the incident James has suffered "intense social criticism", threats to his own safety and had lost his job because his employer could not guarantee his safety.

James was fined $3,500 and his request for a spent conviction was denied.

His co-accused Harrison Angus McPherson, 19, who was the man filmed in the video, had his case adjourned until May for further legal advice.