Why is the plural of reindeer sometimes "reindeer"?
Wiktionary has the following etymology:
From Middle English, from Old Norse hreindýri (“reindeer”), from hreinn + dýr (“animal”).
while the etymology of deer doesn't have any mention of Old Norse, so I assume knowing about the pluralisation of "deer" (from Why is the plural of “deer” the same as the singular?) isn't sufficient to explain this.