The last tile in the world tileset (*TILES.BIN) never actually appears in the game, which led me to open a discussion about this supposed 'mega hamster of doom'.
Johnny Wood pointed out that it might actually be Spuds MacKenzie:
I could be wrong, but it looks like Spuds Mackenzie to me. Spuds was Budweiser's marketing campaign icon around that time period. [...]
I have never seen the tileset for the original apple release, but if that last tile is in there as well, then it couldn't possibly be Spuds.
On 20th April 2019, Dr. Cat confirmed that the tile in fact "would be Spuds McKenzie, from the popular Bud Light ads of the 1980s".
Other theories about the tile included the possibility of it being an upside-down rocket, someone's pet dog, or a hamster dressed like Merlin.
Prescient Dragon's U1 General Info page mentions some places and people which appear in later Ultimas.
Ultima 1 inaccuracies [English | German] can be found at Hacki's Ultima Page [English | German].
The town that we know as "Britain" is misspelled as "Britian" (see screenshot above).
The monster "Minotaur" is misspelled as "Minatour" (spotted at the Enemies page of the RPGClassics Ultima 1 Shrine).
The Rope & Spikes weapon and the Steal spell are remnants of the Apple II version with no particular use in the PC remake.
The Ultima Codex released a scan of a printout of some Ultima 1 source code, with handwritten notes. Check it out at its Origin Gallery entry, or read the news about it.