There's the Krokodil, and now I'm seeing mention of the German Black Swan. Are there others too? And are they all German?
The Amtrak HHP-8's were often called "hippos" by folks here in the States, it was not an official title though The Baldwin DR-12 was known as the "centipede"
The Class 37/9 conversions in the UK were known as "Slugs" due to the extra weight added for more adhesion when hauling heavy steel trains. Class 24's and 25's were known as "Rats" with the Scottish Class 26's and 27's affectionately known as "McRats".
I wouldn't have thought this is what you were looking for, but I am sure we had a 91 named after the guide dogs in the 90s, I don't think the name made it into the 00s and couldn't tell you the exact loco off the top of my head I'm afraid.
We had the CC 1100 called “centipede”, it was an electric shunting locomotive. Also the BB 1-80 named “goat” because… the first notch was quite quick Blue whale for the X73500, beef nose for the VU B6Dux, beef for the A1A A1A 68000, pig nose for the 2D2 5500 and I forgot some.
The region I come from didn't call them "Krokodil". We called them "Eisenschwein" - Iron Pig. Still, an animal name.
Two more that come to mind: There’s the Hamster (hamster) and Grinsekatze (Cheshire Cat). Both are called so because of the shape of their front. Br (1)442 “Hamster” Source Br 1440 “Grinsekatze” Source
We have the British Rail class 89 'Badger'. I think that's what we called it. I think it was the predecessor to the class 90.
The 90 was a development of the Class 87 (in fact it originally was to be the class 87/2, the 89 was a competing design from Brush Traction that was not selected, so the BREL Class 90 became the chosen replacement for the AL Series locomotives instead.
For some reason, I thought I had already posted a thank you to the posters who have replied, but apparently I hadn't. So thank you! I'm enjoying these.
Speaking of Slugs, This here is a yard slug used in switching operations in the USA (Its behind the NS 3004 btw)
The Dutch Railways (Nederlandse Spoorwegen) Mat '54 is called Hondekop, Doghead: Some other trains in the Netherlands are called: mouse-nose (Muizeneus, Mat '46), wasp ('De Wesp' ICNG) and some even call Mat '64 Monkeyhead (Apekop).
The Pesa Link (e.g. German class 632) is called Regio Shark. https://en.wikipedia.org/wiki/Pesa_Link The Skoda 7Ev is called the Regio Panther. https://en.wikipedia.org/wiki/Škoda_7Ev And the Czech Class 471 is the City Elephant. https://en.wikipedia.org/wiki/ČD_Class_471