The frills on either side of its head have cells that generate electricity when exposed to sunlight.
(From Bulbapedia) Helioptile is a small, lizard-like Pokémon. It has a tapering tail, short legs, and small feet with three pointed toes on each foot. Its body is pale yellow, while its feet and the tip of its tail are black. The top of its head is black as well with a small triangular yellow patch between the eyes. It has two long, black pointed frills with yellow tips hanging from the sides of its head, which can stiffen and unfold. These frills have cells which generate electricity when exposed to sunlight. It has large eyes with blue irises and white pupils. Helioptile can generate its own energy from sunlight, so it can thrive without food; it will still eat if needed. It lives in deserts. It and its evolution, Heliolisk are the only known Pokémon capable of learning Electrify.
(From Bulbapedia) Helioptile is probably based on frilled-neck lizards, though its frills also resemble those found on secret toad-headed agama. It may also be based on the spotted salamander, a salamander that can photosynthesize. Its ability to learn Surf may be a reference to the common basilisk, a reptile capable of running on water for short distances. Like what its German name suggests, Helioptile may be based on the iguana too. Additionally, it shares few resemblances to the leopard gecko.
(From Bulbapedia) Helioptile is a combination of ἥλιος helios (Ancient Greek for sun), heliophile (organism attracted to large amounts of sunlight), and reptile.