An eland is a type of African antelope known for its spiraled horns. You'll see it clued in straightforward ways, but keep on the lookout for clues mentioning its African or Serengeti location, or occasionally to zoos. Pay attention to horn descriptions and related creatures like gazelles and impalas. For some more brainteasers that'll test your elastic thinking, try to find the missing words in these puzzles.
Oleo is a somewhat old-fashioned word for butter or margarine that you'll sometimes see in recipes. Look for clues that reference butter, margarine, or cooking oil. It can be clued in such ways to make you think outside the box, as in "it may be on a roll" or "sub in a tub." You'll also see it simply as "toast spread," or "butter alternative."
Eider is great crossword puzzle fill because it has three vowels in addition to two common consonants. Usually, you'll hear about down pillows and not comforters filled with eider, which is a type of duck that has soft feathers. You'll see clues related to pillow stuffing, types of ducks, waterfowl and even clues like "pond denizen" or "where to get down."
13 Words That Will Help You Solve Almost Any Crossword Puzzle, Source:https://www.rd.com/culture/hardest-crossword-puzzle-words/