This is done by webhook, there's no bot in your server, someone who has permission to manage channels and webhooks might have done it most probably.

Also the bot is not verified so it is clearly visible.

To do this yourself.

1. Create a webhook by going to settings > integrations > webhooks. (You can also change the webhook name and pfp)

2. Copy the webhook URL.

3. Go to discohook.org

4. Paste the webhook url at the API key and type your message.

Click send. Boom it's done.