On the official Italian Windows XP's Development Page on Wikipedia, there's is a curiosity section who says:

Whistler nella sua evoluzione ha cambiato logo ed interfaccia grafica. Inoltre, la sua interfaccia grafica fu copiata in Windows Tiger, una versione di Windows 9.x che si doveva chiamare Windows Me SE, ma mai rilasciata

Translates to

Whistler during his evolution changed logo and graphic interface. Also, the Whistler's graphic interface was copied in Windows Tiger, a version of Windows 9x that was should be called Windows Millenium Edition SE, but was never released.

Fake or not? You Decide.

That's actually a fake.

There is this source listed in that page, take a look at it. https://betanews.com/2000/06/21/microso ... ows-tiger/
I don't really know if that source is reliable or not. I've never heard of all of this.

Which directs to The Register, which even doubts it.

So putting aside the sensational news site, there is no hard evidence to support it.

All that's seemingly known about Tiger is that it was some project that Microsoft worked on. That's, as said, no guarantee it was a codename for an operating system, let alone if it had even left any sort of planning stages and went into development. Now, this is speculation on my part, but if Microsoft's past cat-based codenames are any indication, Tiger could have been a planned component of some sort for ME, as certain (planned) components of Windows 95 had been codenamed "Cougar", "Jaguar" and "Panther". Microsoft codenames that fall in some sort of line tend to be consistent, although exceptions to that certainly do exist.