Apple Watch series 4 timer

The native Timer on the Apple Watch is part of the watchOS, and cannot be deleted from the Watch. Go to the Apple Watch app on the iPhone, then to App Layout. Check on there to see if you can located the app icon there. It is orange, and is an almost complete circle with the hand pointing to about 11:00.

You can also try resetting the watch to see if it shows back up. Press and hold the Digital Crown and power button together until you see the Apple logo and then release. If that does not do it, I would un-pair the watch, power cycle it and the phone and then pair them again.

I was unable to find the Timer you are referring to in the App Store, but I did come across this statement, which sounds like a good replacement:

MultiTimer is a simple, free app that allows you to set multiple timers on the Apple Watch. You must create the timers on the iPhone, but once created, you can start and stop them entirely using your Apple Watch. ... You can tap and hold one; you'll be taken to a screen where you can decide the parameters for your timer.