I am new to D&D. I have always liked mimics in all sorts of games like Enter the Gungeon or Terraria. Can my character's race be a mimic?
A changeling can alter its physical appearance with a thought. While this can be used to deceive others, it is a natural form of expression for the changeling.
From my experience of mimics, their ability to imitate other objects relies on their ability to stay still, which I don't believe is viable for a PC.
Note: This race is not part of the core rules, so speak to your DM to see if they are willing to let you play this race beforehand.
On homebrew races
You can technically play D&D as whatever race you like as long as the rest of your group is accepting of a homebrew you've developed or found. There are some guidelines for how to create a custom race in the Dungeon Master's Guide beginning on page 285.
That being said, I strongly encourage against doing this as someone new to D&D. It is very difficult to develop homebrew that works well alongside official options without substantial experience playing and running the system. I would recommend playing a few campaigns with official options and then exploring homebrew down the line once you understand a bit more about how the game works.
On non-humanoid races
The game of D&D is also fairly predicated on some underlying conventions in its player characters.
One of them is that the characters are humanoid (or roughly so); this makes it easier for the GM to incorporate the characters into the setting as the typical setting sees humanoids fill the bulk of tribes, cities, and nations. This doesn't mean nonhumanoids are impossible to make work, but it would change some of the presumptions built into the rules. When new to a game, it is usually best to adhere to the gist of the game's design until you understand how the game works a bit more.
Another is the idea of agency. Since real people are controlling the characters behind the scenes, it is best to work with a character of at least average intelligence and capacity. Experience role players may be able to enjoy and convey characters with minimal capacity for language and even characters with only instinctual behavior, but it is difficult to make them compelling. The mimic in D&D has only 5 intelligence and speaks no languages, putting it above the intelligence of a wolf, and just below the intelligence of an ape. Again, this is a character that could be conveyed in a compelling way, but not something I would recommend for a new player.
Your DM would need to invent a custom mimic race.
While D&D 5th edition has no standard rules for playing monsters as PCs, it does have rules for creating new character races under Dungeon Master's Guide p.285, Creating New Character Options.
Technically, the DM can create any kind of new race, even if it's not humanoid. The only guideline is that it must be balanced against standard player options, so it can't be hugely more or less powerful than any other existing race in the game.
D&D's standard mimic creature stats (Monster Manual, p.220) would be too powerful as a PC race. Its base bite damage is stronger than anything a human fighter can wield, and its ability to disguise as any object with perfect success is better than a rogue's ability to hide.
However, representing the mimic of Enter the Gungeon or Terraria—a creature which looks like a chest and can bite—as a balanced PC race in D&D would be fairly easy.
Personally, I would stat up the mimic as a custom warforged subrace. That race normally represents a humanoid built of of wood and/or metal or stone, but you could re-fluff that as chest-shaped.
Some suggested abilities (a rough draft only here):
- You have no arms, and can only move 25 feet per round. (Presumably the mimic moves upon some kind of retractable pseudopods. Hopping around slowly like a videogame creature or requiring the party to carry you about would be humorous, but would impede adventuring.)
- You can equip items by swallowing instead of wearing them. You can, of course, regurgitate them at will.
- You have a bite attack dealing perhaps 1d10 base damage. This is more powerful than PCs normally get, but it's necessary to compensate for your lack of arms, in which a normal human PC could wield a two-handed weapon.
- You have proficiency with Stealth. You add double your proficiency bonus when you remain motionless (or Advantage, or whatever works out to be most balanced).