Telegram is testing experimental features with Telegram X, an application which it re-branded from Challegram after the company bought it out recently. Challegram, for the unaware, was an alternative chat client to Telegram that was built on the Telegram Database Library, offering many of the same features. Now that Telegram has officially acquired it, it's using the new alt chat platform to its original application to add in new features that aren't available there, such as a native Night Mode that's not to be confused with the currently available Dark theme.
A native Night Mode isn't the only thing that's included in the Telegram X app. The speed of the app is visibly faster as moving back and forth between menus and chats feel smoother than on the original Telegram app, and animations are visibly smoother and faster too. In the app's description on the Play Store these are referred to as slicker animations, so they should just generally appear more visually quick and appealing. Another feature lets you press and hold on active chats that you have and in doing this you can see a preview of the chat before entering it, which might be useful if you have multiple chats with groups and you aren't sure which one might contain something you're looking for, or if you simply want to see what the most recent messages may have been if there are some that have come in which you haven't checked yet.
One of the more interesting features of Telegram X is that you can adjust the size of your text within the app and in chats, so if you want the text to be bigger so it's easier to read or if you want the text to be smaller so you can see more of it on screen without having to scroll as much, Telegram X will allow this. If you like to share stuff, Telegram has made this pretty simple and fluid, as all you have to do to share something or reply to a person's specific message is slide either left or right on the message which holds the content you want to reply to or share. Sliding left brings up the share menu, while sliding to the right brings up the reply option. The app is available in the Play Store via the button below and it is possible to log into it with your current Telegram account and use it just like you would the regular app, if you're curious to check it out. It's also free just like Telegram, and is likely to have more experimental features added in at some point as it appears this might be the new home for Telegram's test functions before rolling them into the main app.