Loaner and loner are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Loaner is an adjective. It is slang for a possession that someone is allowing you to use for a short time.
Loner is an adjective. It describes a person who spends a lot of time by themselves, or prefers to work alone.
The following story uses both words correctly:
Lana was lovely, but unlikely winner for freshman class homecoming queen. Being a loner, she lacked friends and familiarity among her peers. It took all her courage to ask another girl in class for a loaner dress, because she had none for the homecoming court parade float.