Also the creature I'm adding is represented by an @ in ADOM, how could I make that the creatures symbol in game?
I also want to add dragons with elemental breath attacks (such as frost,acid,lightning, and water) how should I add those?
For the @ symbol its code is 064, you'd put it in the creature tile token (without the two ' ' that go around a symbol). That said the symbol is also used for adventurers and non-playable members of your civ in fort mode (i.e diplomats, merchants, guards, etc.) so it might get somewhat confusing.
Closest I've done for lightning is just a regular but strong blunt projectile (SOLID_GLOB) for ball lightning/lightning bolt-type attacks, and paralysis+blisters+bleeding for lightning streams/novas (trailing/undirected gas attacks), while also adding CRAZED to mechanical creatures (makes them go haywire).
For acid its just liquid that does bleeding+blisters ("melting" exists in-game but its only through heat and acid shouldn't be setting things on fire).
Frost is in a similar boat as lightning - frost stream/novas cause blisters+numbness+speed reduction, and the frost bolt type attack is a sharp rock attack (since usually frost bolts are just a fast-moving, big icicle). While cold damage does exist in DF its a bit iffy to get it working with custom inorganics IMO, and it deteriorates stuff incredibly quickly (also kills things incredibly quickly) if you overdo it.
Water depends on what you're trying to do. Usually for water attacks I just go with scalding sprays/waves (trailing/undirected vapors) that cause bleeding+blisters (no actual heat damage though so they don't set stuff on fire) and pressurized bolts that are just blunt projectiles.